THE MESOBLAST. 



241 



tinuous, forming a sheet of somewhat loosely arranged and 

 usually stellate cells, which at the twentieth hour of incubation 

 has a shape and extent indicated by the strong dotted line, M, in 

 Fig. 107. The two halves of the sheet are continuous with each 

 other across the median line in the region of the primitive 

 streak, PS, and behind it ; but in front of the primitive streak, 



Fig. 109. — A diagrammatic figure of the blastoderm of a Hen's Egg about the 

 twenty-fourth hour of incubation. (In part after Duval.) x 8. 



AD, area pellucida : the part left white is the proamnion, and consists of epiblast and 



hypoblast alone ; in the hinder part of the pyriform area pellucida. covered by the light 



ding, mesoblast is present as well. AK, area opaca. BF, commencing fore-brain. 



M, dotted line indicating the limit to which the mesoblast has spread. MS, mesoblastic 



somite or protovertebra. IfG, neural groove. PS, primitive streak. 



For a more exact view of an embryo of this age see Fig. 110. 



in the region where the embryo is formed, NP, the two halves 

 are separated in the middle line by the notochord, the descrip- 

 tion of which is given on the next page. 



In the later stages, as the embryo appears, the mesoblast 

 sheet spreads rapidly. It does not extend directly in front of 

 the embryo, but grows forwards as two lateral horns (Fig. 109), 

 so that for a considerable time there is, immediately in front of 

 the embryo, a transparent area of the blastoderm, ad, which 

 consists of epiblast and hypoblast only, without any middle 

 layer or mesoblast. This area, the proamnion, remains two- 

 layered until about the middle of the third day of incubation, 

 when the two lateral horns of mesoblast gradually grow inwards 

 to meet each other in front of the embryo. 



R 



