i GASTEULATION 37 



oi' the superficially very different gastrulation-phenomena observable 

 in the various groups. 



With regard to the first of these processes, involution of the sur- 

 face, it must be clearly understood that such appearances as that 

 depicted in Fig. 21, B, point indubitably to the occurrence of a true 

 process of invagination or involution of the surface of the egg. It 

 is necessary to emphasize this as some embryologists are sceptical as 

 to the occurrence of true invagination and believe that a more im- 

 portant part in the formation of the archenteric cavity is played by 

 a mere cleavage, or splitting apart, of the cells which are to form the 

 roof and the floor respectively. Brachet (1903), indeed, goes the 

 length of stating in regard to Lepidosiren and Protopterus, with only 

 the data published by myself before him, that " the first trace of 

 the archenteron is due to a cleavage, the result of which is the 

 formation of a slit " — a statement which is certainly not justified. 



On the other hand it should also be borne in mind that there is 

 no difficulty a priori in the way of admitting that portions of enteric 

 cavity may come to arise secondarily by a process of splitting in the 

 midst of a solid mass of endoderm or yolk-cells. This type of modi- 

 fication in the embryonic development of organs which were originally 

 formed by invagination or evagination is one which occurs quite 

 frequently. Numerous examples of it are mentioned in the course 

 of this volume. 



Before closing this account of the gastrulation of Lepidosiren 

 attention should be drawn to a remarkable and important pheno- 

 menon which has been observed in both Lepidosiren and Protopterus. 

 During the early stages of gastrulation, while the segmentation cavity 

 is widely patent, the small blastomeres in the neighbourhood of its 

 abapical side are seen, where not distorted by pressure from their 

 neighbours, to be approximately spherical in shape. Elementary 

 physics teaches that this is an indication that they are isolated 

 masses — that their protoplasmic .substance is not continuous from 

 cell to cell. In a later stage (Fig. 21, C) however the blastomeres in 

 the region of the segmentation cavity do become continuous with their 

 neighbours and form a coarse reticulum traversing the cavity, the 

 fluid contents of which now fill the meshes of the network. 



The importance of the phenomenon described lies in the fact that 

 here we have an actual case, clearly demonstrable, of isolated em- 

 bryonic elements fusing to form a syncytial reticulum — a type of 

 process which may probably, as will be indicated later, play an 

 important part in the development of the Vertebrate nervous system. 



GASTEULATION IN THE VARIOUS GROUPS OF ANAMNIA 



Lampreys. — The Lamprey (Petromyzon fluviatilis) shows in its 

 gastrulation (Fig. 23) an intermediate condition between that of 

 Amphioxus and that of the more heavily yolked holoblastic forms. 

 The abapical portion of the blastula is yolk-laden and thickened, 



