\ I 



I GASTRULATION 49 



nized as the gasfcrular lip. This lip gradually shifts backwards and, 

 as it does so, undergoes alterations in shape, which differ some- 

 what in different species and even in different individuals of the 

 same species but which in their main features are illustrated by 

 Fig. 30. In its later stages the lip becomes bent or curved back- 

 wards so as to have the shape of a XI or a A (Figs. 29, C, and 30). 



Considerable variation occurs between different individuals in 

 the time of the first appearance of the gastrular lip, and in the 

 Gecko Platydactylus Will (1892) observed a correlated variation in 

 shape. Where it appeared relatively early, the involution had the 

 form of an elongated crescentic groove, while in cases where its 

 appearance was delayed the involution formed 

 a shorter and more rounded opening. A g 



As in other cases the central part of the 

 invagination groove becomes deepened to form ^^ — 



a cavity which is clearly homologous with the 

 main part of the archenteric cavity in, say, a 

 frog. This cavity starts by passing directly 

 inwards, perpendicular to the egg-surface, but ' ^ ' f\ 

 it soon bends forwards and runs parallel to the 

 surface (Fig. 31, C and D). The cavity just * "> 

 mentioned (Fig. 31, D, ent.) being an archenteron 

 the layer of cells lining it corresponds to that f ^ 

 which in the lower forms is called endoderm. 

 It is therefore misleading to replace this by any FlG - 30 - — Successive 



. i . i • . i j- j. j.1 . j.1 l ■ stages of gastrular rim 



other name : to emphasize the fact that they line or j ip M seen in 8ur . 

 the true archenteric cavity it may be advisable face view. A,Cheionia, 

 to speak of the cells in question as the archenteric (Mitsukuri 1896). B, 



j j -t p A i • n , i Platydactylus, (Will, 



endoderm in spite oi the clumsiness of the ex- i892). 

 pression. 



In the meantime the lower layer cells immediately underlying 

 the ectoderm assume a flattened form and become joined together by 

 their edges to form a definite epithelium which may conveniently be 

 termed the secondary endoderm (Fig. 31, C and D, end'). The 

 more deeply situated cells underlying the secondary endoderm 

 remain spherical and are separated by wide spaces forming a seg- 

 mentation or subgerminal cavity. These deeper spherical cells have 

 their numbers constantly reinforced by additional cells which are 

 apparently budded off from the underlying yolk-mass. 



The floor of the archenteron becomes closely apposed to the 

 secondary endoderm immediately beneath it (Fig. 31, D). The two 

 cell-layers fuse, irregular perforations develop in the membrane 

 formed by their fusion, and the result is that the archenteron is 

 thrown into communication with the " subgerminal " cavity (Fig. 

 31, E). Shreds of the partition persist for some time but eventually 

 the two spaces form a perfectly continuous cavity just as happened 

 with archenteric and segmentation cavity in the Gymnophiona. 



The portion of the primitive plate which is embraced by the 

 VOL. II B 



