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shall be used to build up one embryo or two depends on the special relations which these primary cytoplasmic substances sustain to one another.

If the eggs of a frog be placed dry on the surface of a slide with their animal poles uppermost and fertilized in that position by the addition of small quantities of the fluid extracted from the seminal vesicles of a male ; if then another slide be placed on top of them and the two slides clamped together by rubber bands; if when the eggs have divided into two blastomeres the whole preparation be in- verted and left in water in a shallow dish for five or six days tadpoles with two heads or two tails will be developed. The materials in the unsegmented egg are of different specific gravities; the first furrow often (see above) divides them into two symmetrical halves; when the two-cell stage is inverted they tend to rearrange themselves in each cell in the same manner as they would have in the whole egg had it been inverted. Nothing has been added or taken away, yet the altered position of the materials in each cell has led to the forma- tion of two organs where normally only one would have been formed. In the case of the newt's egg a similar procedure leads to the forma- tion of two complete embryos, whilst if the blastula of the newt be constricted longitudinally by a hair a two-headed monster is formed. When a lizard's tail is broken off, if the little regenerating bud which forms at the wounded surface be indented the animal will regenerate two tails instead of one.

Internal Environment. When the higher organs begin to develop we can in many cases prove that the whole course of their growth is governed by what may be called their internal environment, i.e. by influences emitted by other organs.

This may be clearly seen in the development of the common sea-urchin Echinus miliaris. The " echinopluteus " larva of this species is a transparent bilaterally symmetrical free-swimming creature. It is provided with a complete alimentary canal consisting of oesophagus, stomach and rectum, and at the sides of the oesophagus are situated two flattened coelomic sacs. As development proceeds each sac becomes divided into anterior and posterior portions, and the latter move backwards so as to be pressed against the stomach. Still later from the posterior end of the left anterior sac a little bud termed the " hydrocoele " grows out. This is the rudiment of the water vascular system of tubes in the adult. The ectoderm lying over this bud becomes depressed so as to form a sac (the " amniotic cavity ") from the floor of which grow up the spines which will cover the test of the future sea-urchin.

The hydrocoele bud overlaps the front end of the left posterior sac, and from this part of the sac there grow out five pockets from which will be developed the dental apparatus the so-called " Aris- totle's lantern." From the outer wall of the right posterior coelomic sac cells are given off from which are developed a pair of " pedi- cellariae " (pincer-organs) which will be situated on the upper surface of the future urchin. If we now allow * the young larvae at the time the coelom is being formed to grow in hypertonic water, then many of them will develop from the right anterior coelom a second hydrocoele bud. If this bud develops and it does so if plentiful nourishment be supplied to the larva then a right amniotic cavity is formed from the overlying ectoderm, whilst the right poste- rior coelom gives rise to a second Aristotle's lantern. If the develop- ment of the second hydrocoele bud be slow then one or even two pedicellariae may be formed on the right side as in normal larvae, but if it be rapid the formation of pedicellariae may be inhibited altogether; If after the bud has appeared the larva is nearly starved for a time, both this abnormal bud and the normal hydrocoele may remain small and undeveloped and then pedicellariae may be formed on the left side as well as on the right.

We conclude from these facts that the hydrocoele bud tends to inhibit the formation of pedicellariae on its own side of the larva but to cause their production on the opposite side, and we see further that the right hydrocoele bud can totally alter the development of the right side of the larva, forcing the right ectoderm to form an amniotic cavity and the right posterior coelom a dental apparatus.

Another still more striking case of the influence of the internal environment is afforded by the results of experiments performed on the tadpole of the frog. 2 The vertebrate eye consists of two main parts, viz. : (a) the retina, formed as an outgrowth from the brain ; and (b) the lens, formed as a thickening of the ectoderm of the side of the head. If before the lens is formed the skin of the head of a tad- pole be slit open and the retina cut off from the brain and pushed back till it occupies a position in the region of the shoulder or even farther back, and the slit in the skin sewn up, then the tadpole will recover; the cut-off retina will continue to live and grow in its new position, and it will force the ectoderm covering it to form a lens although never in the history of the race has a lens been normally formed in this position. Numerous other similar instances could be

'E. W. MacBride, "The Artificial Production of Echinoderm Larvae with Two Water Vascular Systems," Proc. Roy. Soc. (Lon- don), Series B, vol. xc., 1918.

2 W. H. Lewis, " Studies on the Development of the Eye in Am- phibia, I. The Lens," American Journ. Anal., vol. iii., 1904.

adduced did our space permit of it suggesting the conclusion that in many embryos the primary organs are indifferent material and that the manner in which the secondary organs will develop out of them is fundamentally a matter of their spatial relations.

External Environment. We now approach the subject of the possible influence of the external environment on the course of development. In the earlier article the attention of the reader was called to the fact that development presents itself under two principal aspects, viz. the embryonic and the larval. In the embryonic phase the young organism is sheltered from the external world, either within an egg-shell or in the mother's womb, whereas in the larval phase it leads a free life, using its larval organs to seek its own food and escape its enemies.

It was further pointed out that if we compare two nearly allied animals such as Salamandra alra and Salamandra maculosa, in the first of which development is mainly embryonic whereas in the second it is largely larval, we arrive at the conclusion that the embryonic phase is secondarily derived from the larval phase, since the organs such as gills which are functionless in the embryo are functional in the larva. It was also pointed out that larval organs frequently resemble the adult organs of other animals of simpler and more primitive structure.

On these facts was founded the celebrated biogenetic law first enunciated by Haeckel 3 which affirms that " the embryo in its development recapitulates the ancestral history of the race." It is the law which provides a large part of the fascination of embryological research, but it was vigorously attacked in the earlier article and an effort was then made to show that it is not valid, since it was maintained that whilst it is true that larvae retain ancestral characters, the same is true of adults, and that larvae in their structure are not more reminiscent of the former history of the race than are adults.

Now the outcome of recent investigation has in large measure tended to reinstate the doctrine of recapitulation in its former position of preeminence, to show in fact that recapitulation forms the central thread in every life history, although it has been blurred and deflected by secondary influences, as indeed all believers in the biogenetic law have from the first admitted.

The first point to which we wish to direct the reader's attention is that larval and embryonic phases occur in all life histories. Every animal begins its existence as an egg which is quite incapable of feeding or of defending itself and this egg is always protected by an egg-shell although this shell may be very thin, and no animal upon leaving its early shelter and beginning to seek its own food attains at once the structure of the sexually ripe adult. Hence every animal in the course of its development may be said to pass first through an embryonic and then through a larval phase, although the latter phase may be very short and the difference in structure between the larva and the adult inconsiderable. Now, the larval phase being the later is the most recent addition to the life history and therefore the least likely to be modified by secondary factors; if therefore the biogenetic law be valid, it is the larval phase which will possess most ancestral significance. But in the earlier article attention is called to the fact that the identification of a larva as the representa- tive of an ancestor must always be hypothetical because we have no direct knowledge of what the ancestor of any living animal was like. It behooves us therefore to look a little more closely at the rea- sons which actually do induce us to regard a given stage as ancestral.

First, it has been claimed quite recently that direct experimental proof of the validity of the bicgenetic law has been obtained. Kammerer 4 placed young specimens of Salamandra maculosa which had just completed their metamorphosis in cages the floors and walls of which were coloured differently in different cases. The larva of this species has a skin of a uniform dark-greyish tint, but the skin of the adult is gaily coloured with bright yellow patches on a black background. The salamanders which were confined in cages having a floor of moist yellow loam and walls coloured yellow became yel- lower as they grew to maturity a process which occupies between three and four years. The yellow patches, in a word, increased in number and size and tended to become joined together in bands. Those confined in cages with blackened walls and a floor of black garden earth became darker since the yellow patches dwindled in size. When the salamanders had attained sexual maturity and were allowed to pair, it was found that the offspring of two which had been reared in yellow surroundings, if they continued to live in the

3 Haeckel, Allgemeine Morphologic (1866).

4 P. Kammerer, " Vererbung erzwungener Farbveranderungen. IV. Das Farbkleid des Feuersalamanders (Salamandra maculosa) in seiner Abhangigkeit von der Umwelt," Archiv fur Entwicklungs- mechanik, vol. xxxvi., 1913.