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Rh living parts of these bodies, which are included under the general name protoplasm, as built up of a series of units termed cells, each normally containing a single nucleus and separated from one another by quasi-solid membranes termed cell-walls.

The doubts as to the validity of the concept of the cell, which were raised in the later years of the ipth century, have not been sustained by later discoveries. A more refined technique has enabled us to demonstrate a cell-wall in cases where it was supposed to be absent; and where it really is absent, as for instance in the ectoderm of the Nematode worms, it has been proved that this is a secondary state of affairs, due to the de- generation of a well-developed layer of cells, which in younger stages of the life-history are clearly and sharply delimited from each other. It is true that in many, perhaps in most, cases the cell-walls are perforated so that adjacent cells are connected by bars of protoplasm, but this circumstance in no way invalidates the idea of the cell as the unit of structure.

Scope of Embryology. The lowest grade of animals, termed the Protozoa, do not exhibit cellular structure. Either their bodies are so small that they possess only one nucleus, and in this case they may be regarded as free-living cells; or they contain more than one nucleus and attain a greater size, and then their protoplasm is not divided into compartments in accordance with the distribution of these nuclei.

Some of the largest of the Protozoa such as the extinct genus Nummulites were disc-like in form and attained a size of an inch in diameter; the bodies of these animals were divided into thousands of compartments by calcareous septa. To judge from what we know of the structure of their nearest living representatives they must have possessed numerous nuclei; but these nuclei were not distributed in accordance with the divisions of the protoplasm. Some compartments contained several nuclei, some one nucleus only and many none ; so that true all-structure was absent.

In other cases the protozoon may be described as a colony of small uninucleate forms, connected together either by strings of protoplasm or by stalks springing from a common base. But all these more complex Protozoa are distinguished from the true higher animals or Metazoa by the fact that when reproduction takes place the whole body of the parent breaks up into germs, each containing a single nucleus, whereas in true Metazoa small portions only of the parent's body are set aside for reproductive purposes; in other words, in the Metazoa there is a persistent " soma " or body distinct from the germ-cells. Now of course the development 01 the Protozoa ought to form part of the subject matter of embryology, but in the case of the smaller species it is exceedingly difficult to say which stage corresponds to the adult condition of Metazoa, since reproduction by the division of the mother's body into two, can take place at various periods in the life-cycle, and therefore purely as a matter of convenience it is customary to confine the subject matter of embry- ology to the study of the life histories of the higher animals which exhibit definite cellular structure, in a word to the Metazoa.

Metazoa. If we now examine the development of the Metazoa we find a few cases where, side by side with other methods, reproduction by fission, that is by the division of the mother's body, does actually take place.

Thus in the marine annelid Procerastes described by Allen 1 the mother worm breaks up into groups of one, two or three seg- ments and each of these groups regenerates the missing parts and thus constitutes a new worm. In much more numerous cases an outgrowth of the mother's body, termed a " bud," is produced. The bud consists from the beginning of several tissues, and is slowly moulded into the likeness of the parent and when fully grown separates from it, or in the case of a colonial animal remains con- nected with it and helps to build up a compound organism. Such compound creatures are found amongst the sponges, the Coelenterata, the Polyzoa, and the Aseidians, the last-named group being de- generate allies of the Vertebrata.

The laws of bud-development have not been as clearly elucidated as those of the germ-cells. Development by germ- cells is universal amongst the Metazoa; and in all but two phyla the form in which they appear is remarkably constant. They are of two kinds, viz. male and female, and are normally incapable of development unless they have previously united in pairs to form what are called " zygotes " (Gr. ir/ov, a yoke).

The male cell or spermatozoon consists of a head which is a con- densed nucleus made up of a compact mass of chromatin, and a tail

1 E. J. Allen, " An Autotomy and Regeneration in the syllid worm Procerastes." Proc. Roy. Soc. Land., Series B,vol. xcii., 1921.

which is a vibratile filament, Amongst the nematode worms, how- ever, the male cells. are devoid of filaments and appear under the form of small amoeboid cells, whilst amongst the higher Crustacea (i.e. the shrimps, lobsters and crabs) the tail is replaced by a peculiar vesicle, which under certain circumstances absorbs water and ex- plodes, thus propelling the head forwards and in this way bringing about the union of the two germ-cells.

The Germ-cells. The female cell or ovum (egg) is typically rounded and motionless but it is of very different sizes in different species of animals. These differences in size depend entirely on the varying amounts of food-yolk i.e. reserve material deposited in the cytoplasm ; that is to say, in the extra-nuclear protoplasm. The food-yolk in turn differs in amount according to the extent to which the young organism must grow before it can obtain nourishment for itself. Thus the human egg is only about ! mm. in diameter since at a very early period of its development it becomes attached to the wall of the womb and subsequently draws all its nourishment from that source. The egg of the ostrich on the contrary is one of the largest known, being about 15 cm. in diameter, since it has to provide all the food necessary to build up a good-sized chick.

Eggs which have a very small amount of yolk and in which this is evenly distributed throughout the cytoplasm are termed " aleci- thal";such are the eggs of Hydrozoa, Echinodermata, Brachiopoda and of Amphioxus and Mammalia amongst Vertebrata. Eggs in which the yolk is concentrated at one pole of the egg are termed " telolecithal " ; this pole is termed the "vegetative pole," whilst the opposite pole where the bulk of the cytoplasm is concentrated and where the polar bodies (see below) are given off is termed the " animal pole." The eggs of most Annelida and Mollusca and of Pisces, Amphibia, Reptilia and Aves amongst Vertebrata are telo- lecithal. Eggs in which the yolk is massed in the central part of the egg and is surrounded by a layer of cytoplasm almost free from yolk are termed " centrolecithal." To this class belong the eggs of nearly all Arthropoda.

_ Both types of g-crm-cell before attaining maturity undergo two ripening (maturation) divisions, so that in each case four daughter cells are produced. Whereas in the case of the male germ-cell all four daughters become fully formed spermatozoa, in the case of the female germ-cell only one daughter is converted into the ripe egg; the remaining three are small vestigial cells destined to perish, which are termed the " polar bodies." During the maturation divisions the number of chromosomes in the nuclei of both male and female germ-cells is reduced by one-half (for details of this process see CYTOLOGY). When the spermatozoon enters the egg, the head, which is a condensed nucleus, swells up and assumes the ordinary nuclear structure and is termed the "male pronucleus"; behind it is situated a very active centrosome (see CYTOLOGY) which produces a series of radiating rays termed the " spermaster." The nucleus of the ripe egg is termed the " female pronucleus." The male pronu- cleus approaches the female pronucleus, and the spermaster con- stitutes the groundwork of the first mitotic spindle by becoming divided into two asters connected with one another by longitudinal fibres; this spindle initiates the development of the egg by bringing about the first division of the combined male and female pronuclei and of the fertilized egg (zygote) itself. The tail of the spermatozoon is either left outside when the head penetrates the egg, or if it penetrates the cytoplasm it degenerates there; its remnants can sometimes be detected in one cell of the embryo, up till the stage of 32 cells has been attained, but it takes no part in cell-division and no portion of it is transmitted to any other cell, the conclusion being that it plays no part in the transmission of hereditary qualities.

The nucleus of the zygote, as we have just seen, has double the number of chromosomes which are present in the nucleus of the ripe egg but half of these are of male origin. Every nucleus of the developing embryo therefore inherits from the zygote nucleus an equal number of male and female chromosomes, so that the body of the embryo has with justice been likened to a tissue of which the warp is paternal and the woof maternal.

Parthenogenesis. In the earlier article it was pointed out that the unfertilized egg could be induced to develop by a variety of agencies varying from the addition of a small quantity of butyric acid to the sea- water in which it is placed, followed by exposure to the action of hypertonic (i.e. over-salted) sea-water in the case of echinoderm eggs, to the prick of a pin in the case of the eggs of Amphibia. This is termed artificial parthenogenesis.

In the case of the eggs of the sea-urchin (Echinus') parthenogenesis has been minutely studied by Loeb 2 who has put forward various theories as to the action of the agents which he employed. He imagined that the action of the butyric acid was to start cytolysis, one result of which was the formation of a definite egg membrane, but which if unchecked destroyed the egg, which became resolved into a mass of globules. The exposure to hypertonic sea-water was supposed to arrest this injurious action. This explanation was obviously not applicable to the parthenogenesis of the frog's egg.

2 J. Loeb. Numerous papers summarized in his book Die chemisette

Entwicklung des tierischen Eies (1909).