48 WALTER HEAPE. 



Between the two sections described above, the cord of cells 

 (fig. 33) joins the front end of the mass of cells formed by the 

 union of the epiblast and hypoblast in the middle line ; and 

 where this junction occurs there is distinct evidence of an 

 involution of the epiblast layei'. 



From the front end of the primitive streak a tongue-shaped 

 cord of mesoblast cells is projected forwards into the mass of 

 cells underlying the epiblast in that region, and gives rise to 

 the lighter shaded prolongation of the primitive streak seen in 

 fig. 12. 



I have been unable to find any complete perforation of 

 the blastoderm at this stage of growth, although in a some- 

 what younger embryo and in an older one in which the me- 

 dullary groove is formed, there is no doubt that it exists. 

 I have, therefore, either missed the section in which the 

 perforation occurred in this embryo, or it has been closed up 

 by the rapid production of mesoblast which at this stage takes 

 place. 



Although I have only seen a complete perforation of the 

 blastoderm in one embryo during the primitive streak stage, I 

 have invariably found at the front end of the primitive streak 

 evidence of an involution of the epiblast ; on this account, as 

 well as for reasons which will appear in the sequel, I conclude 

 the front end of the primitive groove is the spot where the 

 perforation of the blastoderm seen in fig. 31 occurs at an 

 earlier and later stage. The cord of cells described in the first 

 section is the front wall of the perforation seen in fig. 31, and 

 the tongue of mesoblast projecting forwards is homologous 

 with the anterior growth of mesoblast also seen in the younger 

 embryo. 



This statement is supported by a study of sections of 

 an area but slightly older than that drawn in fig. 11, and 

 somewhat younger than the one we have been considering. 

 In this area there was no layer of cells underlying the epiblast 

 at the anterior end of the primitive streak, and the behaviour 

 of the forward growth of mesoblast from the front end of that 

 Structure could be more definitely determined. At the front 



