THE PRIMITIVE STEEAK. 239 



shape, rather closely packed together, and situated between the 

 epiblast, e, and the hypoblast, H. 



The primitive streak appears before any trace of the ner- 

 vous or other systems of the embryo has commenced to form. 

 The meaning of the primitive streak has been much discussed, 

 but it is now generally agreed that it corresponds, at any rate 

 in part, to the lips of the blastopore in the frog, which have 

 become lengthened out, and fused together; the primitive groove 

 marking the line of concrescence of the lips of opposite sides of 

 the blastopore. The anterior end of the primitive streak in the 

 chick certainly corresponds to the anterior or dorsal lip of the 

 blastopore in the frog ; but it is not quite clear whether the 

 entire length of the primitive streak is to be compared to an 

 elongated and drawn out blastopore, or whether the hinder part 



Fig. 108. — Transverse section across the blastoderm of a Hen's Egg about the 

 twentieth hour of incubation, the section passing through the primitive 

 streak about the middle of its length (of. Fig. 107). x200. 



E, epiblast. H, hypoblast. M. mesoblast. PG. primitiye groove. PS, primitive 

 streak. 



of it is not rather due to the peculiar method of spreading of 

 the blastoderm, imposed on the chick embryo in consequence of 

 the distension of the egg by the enormous mass of food-yolk 

 which it contains. 



5. The Mesoblast. 



The middle germinal layer, or mesoblast, gives rise in the 

 chick, as in Amphioxus, in the frog, and in other animals gene- 

 rally, to all the connective tissue, vascular, muscular, and 

 skeletal structures, as well as to the urinary and reproductive 

 organs. 



In the chick, the mesoblast cells have a less clearly defined 

 origin than in Amphioxus or in the frog, and are derived from 

 three distinct sources. 



(i) In the hinder part of the blastoderm, some of the cells 



