128 



THE PRIMITIVE STREAK. 



In addition to the cells clearly derived from the epiblast, there 

 are certain other cells {vide fig. 95), closely adjoining the hypoblast, 

 which appear to me to be the derivatives of the cells interposed between 

 the epiblast and hypoblast, which gave rise to the posterior opacity 

 in the blastoderm during the previous stage. In my opinion these 

 cells also have a share in forming the future mesoblast. 



The number and distribution of these cells is subject to not inconsider- 

 able variations. In a fair number of cases they are entirely congregated 

 along the line of the primitive streak, leaving the sides of the blastoderm 

 quite free. They then form a layer, which can only with difficulty be 

 distinguished from the cells derived from the epiblast by slight peculiari- 

 ties of staining, and by the presence of a considerable proportion of large 

 granular cells. It is, I believe, by the study of such blastoderms that 

 Kolliker has been led to deny to the intermediate cells of the previous stage 

 any share in the formation of the mesoblast. In other instances, of which 

 fig. 95 is a fairly typical example, they are more widely scattered. To 

 follow with absolute certainty the history of these cells, and to prove that 

 they join the mesoblast is not, I believe, possible by means of sections, 



and I must leave the reader to judge how 

 far the evidence given in the sequel is 

 ^^ sufiicient to justify my opinions on this 



subject. 



In the course of further growth 

 the area pellucida soon becomes pyri- 

 form, the narrower extremity being the 

 posterior. The primitive streak (fig. 96) 

 elongates considerably, so as to occupy 

 about two-thirds of the length of the 

 blastoderm; but its hinder end in 

 many instances does not extend to the 

 posterior border of the area pellucida. 

 The median line of the primitive streak 

 becomes marked by a shallow groove, 

 known as the primitive groove. 



During these changes in external 

 appearance there grow from the sides 

 of the primitive streak two lateral 

 wings of mesoblast cells, which gradu- 

 ally extend till they reach the sides of 

 the area pellucida (fig. 97). The meso- 

 blast still remains attached to the epi- 

 blast along the line of the primitive streak. During this extension 

 many sections through the primitive streak give an impression of the 

 mesoblast being involuted at the lips of a fold, and so support 

 the view above propounded, that the primitive streak is the rudiment 

 of the coalesced lips of the blastopore. The hypoblast below the 

 primitive streak is always quite independent of the mesoblast above, 

 though much more closely attached to it in the median line than at 



Fig. 96. Surface view of the 



AREA PELLUCIDA OF A CHICK's BLASTO- 

 DERM SHORTLY AFTER THE FORMATION 

 OF THE PRIMITIVE GROOVE. 



pr. primitive streak with primi- 

 tive groove ; af. amniotic fold. 



The darker shading round the 

 primitive streak shews the exten- 

 sion of the mesoblast. 



