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they arc caused to retract by electric shocks the movement ends, showing the conversion of the material into a gel.

Side by side with these observations on living protoplasm have gone renewed observations on its structure by means of more refined fixatives and stains. It has been shown that many of the older preserving fluids which were used to dif- ferentiate the nucleus and especially the chromatin of the nucleus from the cytoplasm, had a destructive effect in dis- solving out many of the constituents of the cytoplasm.

A more refined technique has demonstrated the existence of bodies in the cytoplasm termed " mitochondria," which take the form of fibres or. less frequently, of small oval granules. What their function in the life of the cell is has not been determined. Meves ' supposed them to be bearers of heredity like the chromosomes and described them as dividing into two at the division of the cell, the halves being distributed to the two daughter cells. Later re- searches by Gatenby 2 have failed to confirm this. He finds that the mitochondria are irregularly distributed at all divisions. Be- sides the mitochondria another element of the cytoplasm known as the " Golgi apparatus " has come to light. It is so called because it can be demonstrated only by the Golgi method of preservation and a staining, a method which involves the use of preserving fluids which contain osmic acid, followed by treatment with salts of silver. In young cells this apparatus takes the form of a wreath surrounding the centrosphere or area of clear cytoplasm which envelops the centrosome in the young cell i.e. in the cell just after it has or- iginated by division of the mother cell. During the growth of the cell this wreath dissolves into smaller elements which become scat- tered throughout the cytoplasm interspersed amongst the mito- rhondria. These elements in their typical form consist of disc-like bodies made up of a substance which stains faintly, edged for half their circumference by a rim of stains intensely black with the re- agents which are used to differentiate the Golgi apparatus. The earlier observers failed to note the disc-like form of these elements and fixed their attention solely on the deeply staining rim, which they regarded as a rod and to which they gave the name " dicty- some," but Bowen 3 has brought out clearly the real structure of these bodies. The fact that the Golgi apparatus reacts strongly with osmic acid has been held to prove that it must be partially made up of a substance allied to lecithin, which contains a fat group in the molecule.

The most interesting fact about both mitochondria and the Golgi apparatus is that the elements of both increase in number by trans- verse division, and hence they cannot be regarded as food reserves, or temporary deposits of excreta, but are in some sense elements of the living cytoplasm. What relation they have to the general metabolism of the cell is not yet clear. Efforts have been made to show that in certain cells they are the centres of fat formation, and in pancreatic cells of the characteristic pancreatic secretion but these endeavours have not yet carried conviction to the minds of cytologists. Much further work on these lines is needed before certainty can be obtained. In the formation of the spermatozoa of insects and mollusca the mitochondria and the Golgi apparatus play a very definite part which has been elucidated by Gatenby. 4 The former give rise to a sheath surrounding the base of the tail which forms the middle-piece of the spermatozoon; the latter be- comes largely metamorphosed into the " acrosome " or pointed structure which is attached to the front of the head of the sperma- tozoon, and plays the part of a spear-head when the spermatozoon reaches and penetrates an egg. The remnant of the Golgi apparatus migrates round the head and eventually forms a bead projecting from the tail which is eventually rubbed off. The middle-piece penetrates the egg, but undergoes no growth there: it remains embedded in one cell of the embryo and is eventually absorbed so that the mitochondria can have no function as bearers of heredity. In living cells particles resembling mitochondria 6 and Golgi discs 6 in their shape have been observed, oscillating in the typical Brown- ian manner, whence it is inferred that these bodies really exist as

1 F. Meves and J. Duesberg, " Die Spermatozytenteilungen bei der Hornisse," Archiv f. Mikroskopische Anatomic, vol. Ixxi. (1908); F. Meves, " Die Chondrisomen als Trager erblicher Anlagen," ibid., vol. Ixxii. (1908).

2 J. Bronte Gatenby, " The Cytoplasmic Inclusions of the Germ Cells," Quart. Journ. Micr. Sc., vol. Ixii. (1917).

3 R. H. Bowen, "Studies on Insect Spermatogenesis: I. The History of the Cytoplasmic Contents of the Sperm in Hemiptera," Biol. Bulletin, vol. xxxix. (1920).

4 J. B. Gatenby, loc. cit. ; " On the Origin of the Golgi Apparatus on the Middlepiece of the Ripe Sperm of Cavia, etc.," Quart. Jour. Micr. Sc., vol. Ixv. (1921).

6 M. R. Lewis and W. H. Lewis, " Mitochondria in Tissue Cul- ture," American Jour. Anal., vol. xvii. (1915).

6 J. B. Gatenby, "The Cytoplasmic Inclusions of Germ Cells: VII. Modern Technique of Cytology," Quart. Jour. Micr. Sc., vol. Ixiv. (1920).

such in living protoplasm and are not merely artefacts produced by preserving reagents.

Nuclear Structure. As regards advances in our knowledge of nuclear structure, we may note especially the demonstration of the presence of sex chromosomes in many animals. Already before 1915 the fact was known that in certain cases an odd chromosome was present in the ripening germ cells, that is, a chromosome which in one of the ripening divisions of the germ cells passed without dividing to one pole of the mitotic spindle. But Wilson 7 showed that not only are such odd chromosomes found in the developing sperm cells of many insects but that their presence is related to the determination of sex. The odd chromosome sometimes divides like the rest at the first maturation division, but at the second passes undivided to one pole of the spindle. Sometimes it migrates to one pole in the first division and divides like the other chromosomes at the second: in either case its presence causes the formation of two types of germ cell, one possessing one more chromosome than the other. In the case of these insects the ripening eggs are all found to possess the same number of chromosomes: and this number is equal to that found in the sperm cells which have the larger number. It is clear, therefore, that when the eggs are fertilized with the sperm, two types of fertilized egg should be found; one characterized by having one more chromosome than the other, and these eggs should give rise to animals all of whose cells should have nuclei with a number of chromosomes equal to those found in the fertilized egg from which each one arose. If we now examine the cells of the males, it is discovered that they have nuclei with the smaller num- ber of chromosomes, whereas the cells of the female enclose nuclei with the larger number of chromosomes. Therefore it is clear that the male has been produced by the fertilization of an egg by a spermatozoon devoid of the odd chromosome: whereas a female arises when an egg is fertilized by a spermatozoon containing this chromosome.

Wilson showed further that sometimes the odd chromosome has a mate with which it pairs but which is much smaller than itself. This mate is denominated by Wilson the Y chromosome, whereas the large one which alone is present in the case of many insects is termed the X chromosome. The male germ cell which receives the Y chromosome gives rise to a male when it fertilizes an egg. Still other modifications, which lack of space prevent us enter- ing into, are recorded by Wilson. There has been a diligent search for these six chromosomes in the germ cells of other animals, and enthusiastic investigators have announced their discovery in the spermatids of echinoderms, vertebrates and even of man himself. These cases cannot yet be taken as fully proved, mainly owing to the fact that the number of chromosomes in the nuclei of these spermatids is large, and accuracy of count is difficult.

The earlier conception of the preparation for the mitotic division of the nucleus has been that the chromosomes became arranged in a continuous thread termed the spireme, which then became trans- versely segmented into the characteristic number of chromosomes. This conception has gradually been superseded by a much simpler one, viz. that the chromosomes persist as long looped threads during the resting stage of the nucleus and that these long U-shaped filaments become shorter and thicker as mitosis approaches. To this change of view many workers have contributed, amongst whom we may specially mention Agar 8 and Hogben. 9 The free ends of these U's are attached to a spot on the nuclear wall immediately outside which lies the centrosome. In the preparation for the reduc- ing division of the germ cells, when the chromosomes first become distinguishable they are said to be in the " leptotene " stage. These leptotene threads then approach one another in pairs, and these pairs are termed " zygotene " threads. Each pair fuses to form a single thicker thread known as " pachytene," and in this u-ay the number of chromosomes becomes reduced to one half. Though formerly an end-to-end fusion of corresponding chromosomes was strongly believed in, there is to-day no unequivocal evidence that such an end-to-end unison (known as " metasyndesis ") ever takes place. On the contrary, in a continually increasing number of cases a side- by-side union (" parasyndesis ") has been demonstrated. 10 Lately Hogben has shown that parasyndesis takes place in the cockroach, an insect which has formerly been regarded as presenting the typical case of end-to-end union.

7 E. B. Wilson, "Studies of Chromosomes: I. The Behaviour of the Idiochromosomes in Hemiptera," Jour. Exp. Zoo/., vol. ii. (1905); "Studies of Chromosomes: II. The Paired Micro-chro- mosomes, Idiochromosomes, etc., in Hemiptera," ibid., vol. ii. (1905); " Studies of Chromosomes: III. The Sexual Differences of the Chromosome Groups in Hemiptera," ibid., vol. iii. (1906).

8 W. Agar, "The Spermatogenesis of Lepidosiren Paradora," Quart. Jour. Micr. Sc., vol. Ivii. (1911).

9 L. T. Hogben, " Studies on Synapsis: II. Parallel Conjugation and the Prophase Complex in Periplaneta," Proc. Roy. Soc., Series B, vol. xci. (1920).

10 The union of two sister chromosomes in the prophases of the reducing division is frequently termed " synapsis." We have avoided this term because it has also been used to denote the bouquet con- traction of the chromosomes (see below).