430 WALTER HEAPE. 



times when there is no other superficial evidence of ihe exist- 

 ence of the primitive streak, but in these cases I have invariably 

 found by sections that a primitive streak does exist, but that 

 the mesoblast to Avhicli it has given rise is so uniformly dis- 

 tributed everywhere except at the front end, that it is only 

 there apparent. 



The structure of the primitive streak is different in different 

 parts, to illustrate which I have figured sections (figs. 33 — 36) 

 through various regions of the blastoderm drawn in fig. 12. 



The first section (fig. 33) is takeii through the anterior portion 

 of the primitive streak. A plate of columnar epiblast cells 

 extends across the area ; it is thinner at each edge, but of uni- 

 form thickness elsewhere, except in the middle line, where a 

 keel-like ridge is formed. The upper half of the keel is wide 

 and joins the epiblast, with the cells of which it is continuous, 

 and the lower portion projects into a mass of cells below, but 

 has no connection with them. These underlying cells I will 

 deal with later, and will in this place merely draw attention to 

 the fact that the lower borders of the cells of the keel are 

 sharply marked off from them, and that these somewhat oval 

 cells lying below the keel of epiblast are entirely different, both 

 in shape and character, from the cells above them. 



The second section is taken close behind the first ; it passes 

 through the front end of the primitive groove, and is, I believe, 

 in an analogous position to the point immediately behind the 

 perforation existing in the embryo, of wliich fig. 31 is a longi- 

 tudinal section. 



The epiblast is curved in the middle line constituting the 

 primitive groove, and from the cells of this portion of the epi- 

 blast, mesoblast is produced. 



Immediately below the primitive groove there is no layer of 

 hypoblast to be distingiiished, and here mesoblast is produced 

 from hypoblast cells. Laterally all three layers are distinct, 

 but in tlie middle line they may be said to combine with one 

 another, and in this region, therefore, the middle layer is 

 formed from both epiblast and hypoblast. The former does 

 not here extend beyond the boundary of the embryonic area. 



