3° 



CLEAVAGE AND THE GERM LAYERS 



form the body wall or somatopleure ; their inner walls, with the gut entoderm, form 

 the intestinal wall or splanchnopleure. In the meantime, a dorsal plate of cells, cut 

 off from the ectoderm, has formed the neural tube (anlage of the nervous system), 

 and the notochordal plate has become a cord or cylinder of cells (axial skeleton) 

 extending the length of the embryo. In this simple fashion the ground plan of 

 the chordate body is developed. 



In Amphibia the mesodermal diverticula grow out from the dorsal entoderm 

 as soHd plates between the ectoderm and entoderm. Later, these plates split 

 into two layers and the cavity so formed gives rise to the ccelom. 



Ectoderm 



Entoderm 



Ectoderm 



Blastopore 



Entoderm Notochordal 



plate Remnant of floor AflfVarji 



Fig. 21. — Longitudinal sections of the snake's blastoderm at various stages to show the origin of the 

 notochordal plate (adapted after Hertwig). 



Reptiles. — The same pocket-hke depression in the caudal portion of the 

 blastoderm, which gave rise to the cells of the entodermal layer, now invaginates 

 more extensively and forms a pouch which pushes in between ectoderm and ento- 

 derm (Fig. 2\ A and B). The size of the invagination cavity varies in different 

 species; in some it is elongated and narrow, being confined to the middle Kne of 

 the blastoderm. The floor of this pouch soon fuses with the underlying entoderm 

 and the two thin, rupture, and disappear, thus putting the cavity of the pouch in 



