ORIGIN OF THE MESODERM, NOTOCHORD AND NEURAL TUBE 



35 



layer, with the entoderm, forms the intestinal wall, or splanchnopleure. 

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

 the ectoderm, 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 ob- 

 served in the human embryo, but the primitive streak may be recognized 



Post, opening of notochordal 

 canal 

 Primitive streak 



Ant. persisting portion of 

 Ant. opening of notochordal canal 



notochordal canal 



FIG. 27. Median longitudinal section through the blastoderm of a bat ( Vespertilio murinus) 



(after Van Beneden). 



in later stages (Fig. 77), and there is evidence also of a transient opening, 

 the neurenteric canal, leading from the exterior into the cavity of the primi- 

 tive gut (archenteron). In Tarsius, an animal classed by Hubrecht with 

 the primates, the mesoderm has two sources: (i) From the splitting of 

 ectoderm at the caudal edge of the blastoderm; this forms the extra- 



FIG. 28. Diagrams showing the spread of mesoderm in rabbit embryos (Kolliker). In 

 A the mesoderm is represented by the pear-shaped area about the primitive streak at the caudal 

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



embryonic mesoderm and takes no part in forming the body of the embryo. 

 (2) The intra-embryonic mesoderm, which gives rise to body tissues, takes 

 its origin from the primitive streak as in the chick and lower mammals. 

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

 as in Tarsius. 



