34 



CLEAVAGE AND THE GERM LAYERS 



splanchnic layer, with the entoderm, forms the intestinal wall or.^^^Zowc^w^^''^- 

 The neural tube having in the meantime arisen from the neural folds of the ecto- 

 derm, there is present the ground plan of the vertebrate body, the same in man 

 as in Amphioxus. . 



No stages of gastrulation or mesoderm formation have yet been observed in 

 the human embryo, but the primitive streak may be recognized in later stages 



Post, opening of notochordal 

 canal 

 Primitive streak 



Ant. opening of notochordal canal 



notochordal canal 



Fig. 27.— Median longitudinal section through the blastoderm of a bat {Vespertilio murinus) (after Van 



Beneden) . 



(Fig. 77), and there is evidence also of an opening, the neurenteric canal, leading 

 from the exterior into the cavity of the primitive gut (archenteron). In Tarsius, 

 an animal classed by Hubrecht.with the primates, the mesoderm has two sources: 

 (1) From the splitting of ectoderm at the caudal edge of the blastoderm; this forms 



Fig. 28. — Diagrams showing the extent of the mesoderm in rabbit embryos (KoUiker). In A the 

 mesoderm is represented by the pear-shaped area about the primiti\-e streak at the caudal end of the 

 embryonic disc; in B, by the circular area which surrounds the embryonic disc. 



the e xtra-embryonic mesoderm and t akes no part in forming the body of the embryo. 

 (2) The i ntra-emhryqnic mesoderm, which gives rise to body tissues, takes its o rigin 

 from the primitive streak and knot as in the chick and lower mammals. The 

 origin of mesoderm in the human embryo is probably much the same as in 

 Tarsius. 



