4 6 



SEGMENTATION OF THE FERTILIZED OVUM 



ectoderm, we have the ground plan of the vertebrate body, the same in man as in 

 Amphioxus. 



The origin of the mesoderm in the human embryo is unknown, but in Tarsius 

 it has two sources, (i) The primary mesoderm derived by delamination from the 



Fig. 20. — Diagrams showing the extent of the mesoderm in rabbit embryos (Kolliker). In A the 

 mesoderm is represented by the pear-shaped area at the caudal end of the embryonic area; in B by the 

 circular area which surrounds the embryonic area. 



ectoderm at the caudal edge of the germinal area. This forms the extra-embryonic 

 mesoderm and takes no part in forming the body of the embryo. (2) The 

 secondary or intraembryonic mesoderm, which gives rise to body tissues, takes 



Ectoderm 



Mesoderm 

 Entoderm 



Mesodermal segment 

 Ectoderm 



Archeriteron 



Entoderm 



Mrehenteron 



Neural tube 

 /l/ephrotome 

 notochord 



^Splanchnic 

 mesoderm 



Coehm 



Fig. 30. — Diagrams showing the origin of the germ layers of mammals as seen in transverse section 



(modified from Bryce). 



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

 The origin of the mesoderm in human embryos is probably much the same as in 

 Tarsius. 



The Notochord. — In mammals and in man the notochordal plate is described 



