fHE DEVELOPMENT OP THE MOLE. ' 16S 



cranial flexure, ventral prolongation of the anterior end of the fore-gut, and 

 the hooked anterior end of the notochord is also indicated. The notochord is 

 seen to be continuous at its anterior end with the hypoblast and epiblast at 

 the point where the mouth will eventually be formed. The existence of 

 mesoblast cells between the notochord and the floor of the brain is to be 

 noticed. 



Fig. 50, Stage J. — A median longitudinal section through the hind end of 

 an embryo with fourteen protovertebrse. The allantoic cavity has increased 

 in size, and numerous blood-vessels are seen in its walls. The mesoblast 

 surrounding the allantoic cavity is derived from the primitive streak meso- 

 blast. The relation of the epiblast, hypoblast, notochord, and mesoblast at 

 the front end of the primitive streak is seen to be precisely the same as 1 

 indicated in a diagram (Fig. 50) of my former paper (No. 8). The neurenteric 

 canal is obliterated. 



Fig. 51, Stage J. — A transverse section of a portion of the muscle-plate of 

 an embryo with fourteen protovertebrse. The cells of the muscle-plate 

 {msc. pi.) are extended into long processes which are continuous with the 

 epiblast cells (ep.) lying above them. These processes are commencing 

 muscular fibres. 



Fig. 52, Stage J. — Longitudinal section through an embryo with fourteen 

 protovertebrse. The section is taken through a line about half way between 

 a perpendicular and horizontal longitudinal line, and bisects the muscle-plates 

 and aorta of one side of the body. The hypoblast lining the alimentary canal 

 is cut through slightly to one side of the middle line, and the notochord 

 therefore is not shown. The arched muscle-plate and muscular processes of 

 its cells, the aorta and its dorso-lateral prolongations between the proto- 

 vertebrse are shown. 



