GASTKULATION 



39 



As will be gathered from an inspection of Fig. 24 the gradual 

 covering in of the yolk-cells takes place in an eccentric fashion. On 

 the side opposite to that on which the original involution groove 

 makes its appearance there is comparatively little displacement of 

 the boundary between large cells and small, while on the side where 

 the groove is the displacement is relatively great — from a to d in the 

 diagram. Intermediate points of the boundary between large and 

 small cells are displaced more or less according to their greater or 

 less proximity to the point of original involution. 



As regards the method by which the yolk-cells become covered 

 in, it would appear that the " dorsal " lip 

 of the groove advances over the yolk by 9- 



a process of overgrowth, while at those 

 parts of the boundary where there is no 

 invagination-groove the process is one of 

 delamination. The growth of the dorsal 

 lip is clearly indicated by the outline of 

 the yolk-plug in sagittal sections which 

 indicates distortion by pressure from the 

 dorsal lip. 



It will be realized from what has 

 already been said that the outer lip of 

 the circular groove (Fig. 24, d) is simply 

 the rim of the gastrula-mouth or proto- 

 stoma and that the preceding stages are 

 above all characterized by this rim being 

 incomplete. In other words the activity 

 concerned in the involution of the gas- 

 trular rim is accentuated at one point (a) 

 while it is suppressed to such an extent elsewhere as only to become 

 apparent at a comparatively late stage when the edge of the small- 

 celled region has already spread to a great extent over the yolk-cells 

 by a process of delamination. 



It will also be realized that it is not strictly accurate to speak of 

 the circular area bounded by small cells as representing the gastrula- 

 mouth until it is completely enclosed by the gastrular rim. 



The internal changes which accompany the phenomena just 

 described are illustrated by the sagittal sections shown in Fig. 25. 

 In C the portion of the involution groove which first appeared has 

 become much deepened and runs for some distance parallel to the 

 surface as the archenteric cavity. It is bounded superficially by a 

 completed portion of gastrular wall showing the two primary cell 

 layers, ectoderm and endoderm. 



Some of the yolk-cells round the margin of the segmentation 

 cavity are frequently to be seen, though not in the section figured, 

 to be spreading along the inner surface of its roof, towards the point 

 which was the apical pole of the blastula. 



In the later stages the overgrowth by the gastrular lip, 



Fig. 24. — Diagram to illustrate 

 overgrowth by dorsal lip of 

 blastopore in the Frog. (After 

 Morgan, 1897.) 



The lines a, i, c, d. represent the 

 involution groove at successive stages 

 of development. 



