REPRODUCTION 



129 



invagination of the yolk-filled cells of the vegetal pole, a turning in of 

 cells at the dorsal lip of the blastopore (involution), and a growth of 

 ectoderm down and over the cells of the vegetal pole (epiboly) (Fig. 6.9). 



In all multicellular animals, except sponges and coelenterates, 

 which never develop beyond the gastrula stage, a third layer of cells, the 

 mesoderm, develops between ectoderm and endoderm. In annelids, 

 molluscs and certain other invertebrates, the mesoderm develops from 

 special cells which are differentiated early in cleavage (p. 237). These 

 migrate to the interior and come to lie bet^veen the ectoderm and endo- 

 derm. They then multiply to form two longitudinal cords of cells which 

 develop into sheets of mesoderm between the ectoderm and endoderm. 

 The coelomic cavity originates by the splitting of the sheets to form 

 pockets, and hence is called a schizocoeie. 



In primitive chordates the mesoderm arises as a series of bilateral 

 pouches from the endoderm (Fig. 6.8). These lose their connection with 

 the gut and fuse one with another to form a connected layer. The 

 cavity of the pouches is retained as the coelom, which is called an 

 enterocoeie because it is derived indirectly from the archenteron. The 

 mesoderm in amphibia is formed in part from the endoderm of the 

 roof of the archenteron and in part from the ectoderm and endoderm 

 at the dorsal lip of the blastopore (Fig. 6.9). In birds and mammals the 

 primitive streak which develops on the surface of the developing embryo 

 is homologous to the dorsal lip of the blastopore of lower forms. It is a 

 thickened band of ectoderm and endoderm cells which marks the lon- 

 gitudinal axis of the embryo. At the primitive streak cells migrate in 

 from the surface, proliferate, and form a sheet of mesoderm between 

 ectoderm and endoderm. 



■Bla-stocoele." 



Archenteroii- 



■DT^^i. .... ^*-"'j"^-»' I Involution 



^ Blastopore -^j^^^^.^^^.^^ ^ ^YolKplug ^ 



Notochord- 



Ectod(z.rm- 

 Me-soderrtr 



N^ural plate 

 N<Lural fold 



Entoderm 



E 



Figure 6.9. A-D, Successive stages in gastrulation and mesoderm formation in 

 Amphibia. E, Transverse section of an early neurula stage. 



