MAMMALIA. 223 



seems possible 'to trace a separation between the two parts (fig. 

 139 B) of the mesoblast. 



During the seventh day the primitive streak becomes a more 

 pronounced structure, the mesoblast in its neighbourhood in- 

 creases in quantity, while an axial groove the primitive groove 

 is formed on its upper surface. The mesoblastic layer in 

 front of the primitive streak becomes thicker, and, in the two- 

 layered region in front, the epiblast becomes several rows deep 

 (fig. 139 A). 



In the part of the embryonic area in front of the primitive 

 streak there arise during the eighth day two folds bounding a 

 shallow median groove, which meet in 

 front, but diverge behind, and enclose 

 between them the foremost end of the 

 primitive streak (fig. 141). These folds 

 are the medullary folds and they consti- 

 tute the first definite traces of the em- 

 bryo. The medullary plate bounded by 

 them rapidly grows in length, the primi- 

 tive streak always remaining at its hinder 

 end. While the lateral epiblast is formed 

 of several rows of cells, that of the me- FlG I40 EMBRYONIC 



dullary plate is at first formed of but a AREA OF AN EI GHT DA YS' 



. _, RABBIT. (After Kblliker.) 



Single row (fig. 142, mg\ The mesoblast, argf em bryonic area ;pr. 



which appears to grow forward from the primitive streak, 

 primitive streak, is stated to be at first a continuous sheet be- 

 tween the epiblast and hypoblast (Hensen). The evidence on 

 this point does not however appear to me to be quite conclusive. 

 In any case, as soon as ever the medullary groove is formed, the 

 mesoblast becomes divided, exactly as in Lacerta and Elasmo- 

 branchii, into two independent lateral plates, which are not 

 continuous across the middle line (fig. 142, me). The hypoblast 

 cells are flattened laterally, but become columnar beneath the 

 medullary plate (fig. 142). 



In tracing the changes which take place in the relations of 

 the layers, in passing from the region of the embryo to that of 

 the primitive streak, it will be convenient to follow the account 

 given by Schafer for the guinea-pig (No. 190), which on this 

 point is far fuller and more satisfactory than that of other ob- 



