36 EMBRYOLOGY OF THE LOWEE VERTEBRATES oh. 



clearly in Fig. 22, A, where it is seen that portions of large yolk-cells 

 adjacent to the small-celled area become split off as small cells which 

 are added to that area. It is to this splitting-off process that the 

 name delamination is applied. 



It is clear, then, that in the gastrulation of Lepidosiren three 



Fig. 22. — Portions of sagittal sections of Lepidosiren egg during early stages of gastrulation. 



A, showing process of delamination. The small-celled ectoderm is seen on the left : it is becoming 

 extended by the addition, to its lower edge, of cells split off from the yolk-cells. The latter are 

 recognizable by their larger size and by the larger size of the yolk -granules with which their cytoplasm 

 is laden. B and C showing involution of the surface along the invagination-groove. 



processes are at work (1) Involution of the surface — this is conspicu- 

 ous in the first stages (see Fig. 22, B and C), (2) Overgrowth by the 

 gastrular lip — the " dorsal " lip as it is commonly termed from its 

 ultimate position, and (3) Delamination. These same three factors 

 are at work in the gastrulation of the lower Vertebrates in general, 

 and a clear realizing of their nature is necessary to a comprehension 



