44 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. 



At a stage when the involution groove forms a nearly complete 

 circle a sagittal section presents the appearance shown in Fig. 27, C. 

 The ectoderm is thick and columnar posteriorly, but in front and 

 laterally it thins out into a cubical epithelium which has extended 

 o\vr the whole surface of the egg with the exception of a small 

 area behind the gastrular rim. In the roof of the enteric cavity the 

 boundary between the archenteric portion formed by overgrowth (and 

 probably involution) and that formed from yolk-cells is marked by 

 an abrupt change in the character of the cells which at once become 

 less tall and less columnar. Farther in still the yolk-section of the 

 roof shows marked irregularities of its inner surface and its cells 

 assume a more rounded form. The anterior limit of the blastocoelic 

 portion of the enteric cavity is not, as yet, clearly defined. 



The last section figured (Fig. 27, D) is taken from an egg in which 

 the gastrulation lip forms a complete ring. Consequently the section 

 shows a conspicuous yolk-plug (y.p} within the gastrular lip which, 

 it will be noted, has developed a covering of small fine-grained cells 

 over its surface. The inrollmg of the gastrular lip visible in the 

 section indicates that the enteric roof is growing actively in length 

 though Brauer does not make it clear to what extent the formation 

 of the archenteron is due to this and to what extent to actual involu- 

 tion. Naturally it would be very difficult if not impossible to decide 

 this point definitely without experiments on the living egg. The 

 gastrular opening gradually decreases in diameter (the yolk-plug 

 disappearing from view as it does so) and eventually it closes from 

 before backwards by its lateral lips coming together (Fig. 26) ; its 

 posterior part however remains open as the anus. 



In the foregoing description is given merely a summary of those 

 features in the gastrulation of Hypogeophis which appear to be of 

 importance in relation to the corresponding phenomena of the 

 Amniota : amongst these may be specially mentioned the process of 

 constriction of the gastrular opening, and the double origin of the 

 enteric cavity from archenteron and blastocoele, only its hinder 

 portion being derived from archenteron. 



Another important feature not specifically alluded to in the text 

 but which is indicated clearly by Fig. 26 is that during the process 

 of gastrulation the boundary of the small-celled area is sweeping 

 onwards over the egg's surface. It does this probably by a process 

 of delamination as in Lepidosiren. The important point to notice, 

 however, is that the small-celled boundary is not blocked in its ex- 

 tension onwards by the gastrular lip. The yolk-plug becomes covered 

 wii li small cells and after the ends of the rim have met so as to form 

 ;i complete circle the small-celled region still spreads onwards, so that 

 the slit-like hlaslnj..'n: of later stages lies well within tin- margin of 

 ili> .-mall-celled area. Thus were development modified by the 

 slurring over of the early stages of the inva-iiution Lrroovr so that 

 i his '.nly became apparent at the period when il had assumed the 1'nnn 

 of a longitudinal slit, it would at the time of its first appearance 



