860 RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 



and in his later memoir has entered with considerable detail 

 into the part played by the various layers in the formation of 

 this structure. 



He believes, as already mentioned, that the sickle-shaped 

 structure, which appears according to him at an earlier stage 

 than is admitted by Gerlach, is in the first instance due to a 

 thickening of the hypoblast. At a later stage he finds that the 

 epiblast in the centre of the sickle becomes thickened, and that 

 a groove makes its appearance in this thickening which he calls 

 the "Sichel-rinne." This groove is identical with that first 

 described by Kupffer and Benecke 1 in the sparrow and fowl. 

 We have never, however, found very clear indications of it in 

 our sections. 



In the next stage, Koller states that, in the region immedi- 

 ately in front of the "Sichel-rinne," a prominence appears which 

 he calls the Sichelknopf, and from this a process grows forwards 

 which constitutes the primitive streak. This structure is in main 

 derived from a proliferation of epiblast cells, but Koller admits 

 that some of the cells just above the hypoblast in the region of 

 the Sichelknopf are probably derived from the hypoblast. Since 

 these cells form part of the mesoblast it is obvious that Keller's 

 views on the origin of the mesoblast of the primitive streak 

 closely approach those which we have put forward. 



The primitive streak starting, as we have seen, at the hinder 

 border of the area pellucida, soon elongates till it eventually 

 occupies at least two-thirds of the length of the area. As Koller 

 \loc. cit.} has stated, this can only be supposed to happen in one 

 of two ways, viz. either by a progression forward of the region 

 of epiblast budding off mesoblast, or by an interstitial growth of 

 the area of budding epiblast. Koller adopts the second of these 

 alternatives, but we cannot follow him in doing so. The simplest 

 method of testing the point is by measuring the distance between 

 the front end of the primitive streak and the front border of the 

 area pellucida at different stages of growth of the primitive 

 streak. If this distance diminishes with the elongation of the 

 primitive streak then clearly the second of the two alternatives 

 is out of the question. 



1 Die erstt Entvriek. an Eier d. Reptilim. Konigsbcrg, 1878. 



