into the Growth of the Blastoderm of the Chick. 355 



(ii) 



.X 



-J- X 



FiG. C. (i) Diagram of unincubated Blastoderm. 



(ii) Blastoderm of Chick with seven pairs of Mesoblastic Somites. 



that the primitive streak is converted directly into a part of the 

 embryo, that is to say, the part of the embryo posterior to, and in- 

 cluding the first pair of mesoblastic somites. 



With regard to the area vasculosa, my experiments seem to indi- 

 cate that the part of the blastoderm which becomes area vasculosa 

 is that part which lies on the inner edge of the posterior part of 

 the area opaca of the unincubated blastoderm. It is along this 

 edge where, according to Koller,* a white crescent is always visible. 



Koller, further, asserts that this white crescent is grooved. From 

 this crescent and groove Koller derives the primitive streak and 

 primitive groove by the conversion of the transverse crescent and 

 groove into a longitudinal streak and groove. 



I think that all recent authors are agreed that it is not grooved, 

 and most admit that it has nothing to do with the primitive streak. 



It is, however, quite true that a crescentic whiter area is sometimes 

 visible here, bat in, I think, the majority of cases there is nothing of 

 the kind to be seen. 



When it is present a sagittal section of the hinder part of the 

 blastoderm seems to reveal its nature. In such a section a mass of 

 inner-layer cells, which would, perhaps, be more properly described 

 as a band of yolk containing numerous nuclei, although quite sharply 

 marked off from the underlying yolk-mass, can be detected. This 

 area corresponds in position to that part of the blastoderm from 

 which, according to experiments made with bristles, the area vas- 

 culosa is derived (figs. D, E). 



A sable hair inserted in the yolk beyond the limits of the blasto- 



* " Beitrage zur Kenntniss des Hiihnerkeims im Beginne der Bebriitung," ' Sitz. 

 Akad. der Wissensch.,' Wien, vol. 80, 1379. " TJntersuch. iiber d. Blatterbildung 

 im Hiilmerkeim," ' Arch. f. mikr.-Anat.,' vol. 20, 1881. 



