X.] 



THK MESOBLAST. 



Fig. 102. 

 A. 



321 



Two Teausveesb Sections through the Embryonic Area 

 OP AN Embryo Babbit of Seven Days. 



The embryo has nearly the appearance represented in Fig. 100. 



A. is taken through the anterior part of the embryonic area. 

 It represents about half the breadth of the area, and there is no 

 trace of a medullary groove or of the mesoblast. 



B. is taken through the posterior part of the primitive 

 streak. 



ep. epiblast ; hy. hypoblast. 



lateral epiblast is formed of several rows of cells, that of 

 the medullary plate is at first formed of but a single 

 row (Fig. 104, mg). 



The mesoblast and notochord. The mesoblast in 

 mammalia has, as in the chick, a double origin, and the 

 details of its development appear to resemble essentially 

 those in the chick. It arises (1) from the epiblast of 

 the primitive streak ; this has been already described ; 

 (2) from the primitive hypoblast in front and at the 

 sides of the primitive streak. The latter is known as 

 hypoblastic mesoblast, and as in the chick appears to 

 originate as two lateral plates split off from the primi- 

 tive hypoblast. These two plates are at first continuous 

 P. & B. 21 



