34 EMBRYOLOGY OF THE LOWER VERTEBRATES oh. 



for the invaginative activity to be diminished along the region 

 corresponding to the posterior part of the gastrular rim of Amphioxus. 



(3) Lepidosiren. — The first sign of gastrulation is afforded by the 

 appearance, a short distance to the abapical side of the equator, of 

 a latitudinally arranged row of small dimples or depressions of the 

 surface which soon become joined up to form a continuous in- 

 vagination-groove. This (Fig. 20, A and B) may extend through 

 about one-third of the circumference of the egg but in contrast with 

 what happens in Polypterus the groove, instead of increasing in 

 length, becomes shorter, flattening out and disappearing at its 

 two ends. 



The final stage is seen in Fig. 20, E, where the gastrular lip is 



_.. ' A "W*T-" B ^™»T c 



Flu. 20. — Illustrating the process of gastrulation in Lepidosiren. 



Fig. A is a side view, the egg being orientated so as to correspond with the figures of Ampliioxm; and 

 Polypterus, the large-celled yolky abapical portion of the egg being above and towards the right hand. 

 Figs. B to E are views looking directly at the gastrular rim (dorsal lip of the blastopore), or in the 

 case of E directly at the completed blastopore. Consequently, as the gastrular rim is during these 

 phases of development not stationary, the views B to E are not orientated morphologically in exactly 

 the same way. 



short, and curved into a crescent, forming the dorsal boundary of 

 the blastopore. 



At this stage the large yolk-cells with their conspicuous salmon 

 colour have been completely covered in by small cells — a condition 

 that has been brought about through the agency of two distinct 

 factors (1) the process of overgrowth with which we have already 

 become familiar and (2) a new process to which the name de- 

 lamination is given. 



As is shown by the sections drawn in Fig. 21, the yolky or 

 abapical portion of the blastula-wall is in Lepidosiren, as it was in 

 Polypterus, far too bulky to be involuted bodily as was the case in 

 Amphioxus. Again the enclosing of the yolky mass by the gastrular 

 lip growing over it as in Polypterus is rendered impossible by the 

 fact that the gastrular lip is normally here never completed to form 

 an entire circle. It is, as has been explained, restricted to a com- 

 paratively small linear extent. 



This small persisting portion of gastrular lip probably does 

 advance over the surface of the yolk by a process of overgrowth, 

 giving rise in this way, just as in Amphioxus, to what will become 

 the dorsal wall of the embryo with its central nervous system and 

 notochord. That this is the case seems to be indicated by sagittal 

 sections through eggs cut in celloidin while still contained within 



