III.] SUMMARY. 75 



tion of what is called the vascular area, the outer border 

 of which marks the extreme limit to which the meso- 

 blast extends. 



The changes then which occur during the first day 

 may thus be briefly summarized : 



(1) The hypoblast is formed as a continuous layer 

 of plate-like cells from the lower layer of the segmenta- 

 tion spheres. 



(2) The primitive streak is formed in the hinder 

 part of the area pellucida as a linear proliferation of 

 epiblast cells. These cells spread out as a layer on 

 each side of the primitive streak, and form part of the 

 mesoblast. 



(3) The primitive groove is formed along the axis 

 of the primitive streak. 



(4) The pellucid area becomes pear-shaped, the 

 broad end corresponding with the future head of the 

 embryo. Its long axis lies at right angles to the long 

 axis of the egg. 



(5) The medullary plate with the medullary groove 

 makes its appearance in front of the primitive groove. 



(6) The primitive hypoblast in the region of the 

 medullary plate gives rise to an axial rod of cells forming 

 the notochord, and to two lateral plates of mesoblast. 

 The innermost stratum of the primitive layer forms the 

 permanent hypoblast. 



(7) The development of the head-fold gives rise 

 to the first definite appearance of the head. 



(8) The medullary folds rise up and meet first in 

 the region of the mid-brain to form the neural tube. 



(9) By the cleavage of the mesoblast, the somato- 

 pleure separates from the splanchnopleure. 



