36 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. 



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 



Fi.. 'I'l. Portions of sagittal sections of Lepidosiren egg during early stages of gastrulation. 



A, showing process ot'delammat ion. Tin- small-celled ectoderm is seen <>n the left: it is becoming 

 extended by Hi-- addition, to its lower ede f of cells split oH' from the yolk-cells. The latter are 

 recognizable by their larger sixe and by the larger size of the yolk -granules \\ith \\liich their r\ toplasm 

 is laden. B and C showing involution of the surface along the inva.dnation-xroov- -. 



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

 ous in the first stages (see Fig. 22, I'. ;m<l C), (2) Over-n.wi h l.y the 

 gaafrular tipthe "dorsal" kp as it is connnonly termed 1'nmi its 

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

 are at work in lli<- L;.ist i-ulation nl' the ].\vci- N'crtrl. rates in ^,-1 

 and a oleai iv.-ilixiu^ of their nature is necessary to a 



