30 CLEAVAGE AND THE GERM LAYERS 



the blastodermic vesicle (Figs. 17, 74 A and 75). The entodermal layer 

 is usually said to arise by splitting, or delamination. Recent careful 

 work, however, shows that in some mammals, at least, the entoderm cells 

 originally lie within the inner cell mass and reach the inner surface by 

 migration (Hartman, 1919). 



ORIGIN OF THE MESODERM, NOTOCHORD AND NEURAL TUBE 



Amphioxus and Amphibia. The dorsal portion of the entodermal 

 sheet, which forms the roof of the archenteron in Amphioxus, gives rise to 

 paired lateral diverticula, or ccelomic pouches (Fig. 20). These separate 

 both from the plate of cells in the mid-dorsal line (which forms the noto- 



n.g. 



ccel. p. 



FIG. 20. Origin of the mesoderm in Amphioxus (after Hatschek). X about 425. n.g., 

 Neural groove; n.c., neural canal; ch., anlage of notochord; ccel. p., ccelomic pouch; ect., ectoderm; 

 ent., entoderm; a/., cavity of gut; cce., ccelom or body cavity. 



chord}, and from the entoderm of the gut, and become the primary meso- 

 derm. The mesodermal pouches grow ventral and their cavities form the 

 ccelom, or body cavity. Their outer walls, with the ectoderm, 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, instead of mesodermal diverticula solid plates grow out 

 from the dorsal entoderm between the ectoderm and entoderm. Later, 

 these plates split into two layers and the cavity so formed give rise to the 

 ccelom. 



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

 the blastoderm, that gave rise to the cells of the entodermal layer, now 

 invaginates more extensively and forms a pouch which pushes forward 



