122 F. M. BALFOUR AND F. DEIGHTON. 



the area pellucida. This process is the primitive streak, and it 

 is formed, like the sickle, of proliferating epiblast cells. 



Roller ^ described the sickle and the growth forwards from it 

 of the primitive streak in surface views somewhat before Gerlach ; 

 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 ^ 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 immediately 

 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 

 KolWs 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 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 difi'erent stages of growth of 

 the primitive streak. If this distance diminishes with the elon- 

 gation of the primitive streak then clearly the second of the two 

 alternatives is out of the question. 



We have made measurements to test this point, and find that 

 the diminution of the space between the front end of the 



1 "Beitr. z. Kenntniss d. Hiihnerkeims im Beginne d. Bebriitung," 

 ' Sitz. d. k. Akad. Wiss.,' iv Abth., 1879. 

 ' 'Die erste Entwick. an Eier d. Reptilien.' Konigsberg, 1878. 



