xviii SUMMARY 657 



mode of life, and then the adult stage, for the evocation of which the 

 appropriate stimulus has ceased, drops out. As is well known, this 

 lost adult stage can be evoked in the Axolotl when the proper 

 stimulus is discovered and artificially apphed. 



From considerations of the secondary factors which modify larval 

 hfe, we pass to the factors which modify embryonic hfe. Of these 

 the most important is food. Food is supphed to the embryo either 

 in the form of yolk grains embedded in the cytoplasm of the egg, or 

 of maternal secretions. These two forms of food give rise to different 

 kinds of modifications. To take the yolk first. Its presence in any 

 quantity slows down cell division, as Balfour (1881) has pointed out, 

 and in extreme cases abolishes it altogether, leaving only division of 

 nuclei. It causes the number of cells produced to be smaller, and the 

 size of the individual cell to be larger, and so it renders such processes 

 as the formation of pouches, or of folding, impossible, and they are 

 replaced in the yolky embryo by processes of solid prohferation of 

 cells. Ordinarily speaking, yolk is stored in the ceUs which will 

 afterwards form part of the gut, but in certain cases, as in certain 

 Echinoderm embryos, it is more widely distributed; and then the 

 most profound modification in early development can take place, the 

 whole of the archenteric wall may be directly converted into coelom, 

 and the true gut formed later as an outgrowth from this. 



Maternal secretions are in most cases absorbed through the 

 ectoderm, and the effects of this change of function on this layer are 

 profound. The extraordinary spongy ectoderm of Peripatus capensis 

 is due to this, but the most striking instances of the effects of this 

 kind of food on the development of the embryo are found amongst 

 Mammaha, and will doubtless be dealt with in the third volume of 

 this work. 



Finally, it must be remembered that, in the embryonic phase 

 of development, the functional correlation of the development of 

 the various organs necessary to the larva, as to every free-hving self- 

 supporting organism, can be profoundly altered without impairing the 

 end of development — viz. -the successful attainment of the adult form. 

 The structure of the embryo, at any period, is the outward and visible 

 expression of the co-working of independent processes, held together 

 by a very loose rein. A beautiful instance of this has been given by 

 Jenkinson (1906). In the embryo of Urodela the lower part of the 

 blastopore remains continuously open and eventually forms the anus. 

 In the Anuran embryo this part of the blastopore closes, but the anus 

 is formed later as a new perforation in identically the same place. 

 Ifow, by allowing the Anuran egg to develop in solutions of certain 

 salts, the blastopore can be caused to remain open and the egg then 

 develops after the Urodele manner. The question as to whether the 

 blastopore persists or not is, therefore, merely a question of the value 

 of the differential equation connecting the rates of growth of the 

 archenteric walls, and the rate of expansion of the archenteric 

 cavity. 



VOL. I 2 u 



