154 



THE PRIMITIVE STREAK. 



only, but in the region of the streak itself the structure of the 

 blastoderm is greatly altered. The most important features in 

 it are represented in fig. 95. This figure shews that the median 

 portion of the blastoderm has become very much thickened (thus 

 producing the opacity of the primitive streak), and that this 

 thickening is caused by a proliferation of rounded cells from the 

 epiblast. In the very young primitive streak, of which fig. 95 is 

 a section, the rounded cells are still continuous throughout with 

 the epiblast, but they form nevertheless the rudiment of the 

 greater part of a sheet of mesoblast, which will soon arise in 

 this region. 



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 peculiarities 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 inter- 

 mediate 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 wide- 

 ly scattered. To follow with absolute cer- 

 tainty 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 

 sufficient to justify my opinions on this 

 subject. 



FIG. 96. SURFACE VIEW OF 

 THE AREA PELLUCIDA OF A CHICK'S 

 BLASTODERM SHORTLY AFTER THE 

 FORMATION OF THE PRIMITIVE 



GROOVE. 



pr. primitive streak with primi- 

 tive groove ; of. amniotic fold. 



The darker shading round the 

 primitive streak shews the exten- 

 sion of the mesoblast. 



