110 THE FKOG. 



The mesoblast (Fig. 56, M) thus arises in the frog as two 

 lateral sheets of cells, split off from the outer surface of the 

 hypoblast and yolk-cells. The two sheets very early become 

 continuous with each other in the mid-ventral plane, but are 

 separated dorsally by the notochord, which is formed, indepen- 

 dently, from the hypoblast in the mid-dorsal region. 



At intervals along their length, the mesoblast sheets remain 

 for a time attached to the hypoblast along the dorsal surface of 

 the mesenteron, not far from the median plane ; and, at these 



FIG. 56. A transverse section through the middle of the length of a Frog 

 Embryo at about the stage represented in Fig. 55. x 25. 



CH, notochonl. E, epiblast. HM, pouch-like diverticulum of the hypoblast into 

 the mesoblast. M, mesoblast. NG, neural groove. NP, neural plate. T, mesenterou. 

 Y, yolk-cells. 



places, slight pouch-like diverticula from the mesenteron (Fig. 

 56, HM) may be seen extending into the mesoblast^ sheets. It 

 has been suggested by Hertuig that these diverticula are pos- 

 sibly indications of a mode of origin of the mesoblast as hollow 

 diverticula from the mesenteron, such as occurs in Amphioxus 

 (</. Figs. 24 and 28, CE). 



5. The Blastopore and the Primitive Streak. 



The blastopore, or anus of Rusconi, has been defined above as 

 the circular aperture in the epiblast which is filled up by the 

 yolk-plug (Figs. 52 and 58, A) ; the lip of the blastopore and 



