RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 859 



The cells of the primitive streak between the epiblast and 

 the hypoblast are without doubt mesoblastic, and constitute the 

 first portion of the mesoblast which is established. The section 

 of these cells attached to the epiblast, in our opinion, clearly 

 originates from the epiblast ; while the looser cells adjoining 

 the hypoblast must, it appears to us, be admitted to have their 

 origin in the indifferent cells of the embryonic shield, placed 

 between the epiblast and the hypoblast, and also very probably 

 in a distinct proliferation from the hypoblast below the primitive 

 streak. 



Posteriorly the breadth of the streak of epiblast which buds 

 off the cells of the primitive streak widens considerably, and in 

 the case of the blastoderm with the earliest primitive streaks 

 extends into the region of the area opaca. The widening of the 

 primitive streak behind is shewn in Ser. B, 3 ; Ser. c, 2 ; and Ser. 

 E, 4. Where very marked it gives rise to the sickle-shaped 

 appearance upon which so much stress has been laid by Koller 

 and Gerlach. In the case of one of the youngest of our blasto- 

 derms of this stage in which we found in surface views (PI. 45, 

 fig. L) a very well-marked sickle-shaped appearance at the hind 

 end of the primitive streak, the appearance was caused, as is 

 clearly brought out by our sections, by a thickening of the hypo- 

 blast of the germinal wall. 



There is a short gap in our observations between the stage 

 with a young primitive streak and the first described stage in 

 which no such structure is present. This gap has been filled up 

 both by Gerlach and Koller. 



Gerlach states that during this period a small portion of the 

 epiblast, within the region of the area opaca, but close to the 

 posterior border of the area pellucida, becomes thickened by a 

 proliferation of its cells. This portion gradually grows out- 

 wards laterally, forming in this way a sickle-shaped structure. 

 From the middle of this sickle a process next grows forward 

 into the area pellucida. This process is the primitive streak, 

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



Koller 1 described the sickle and the growth forwards from it 

 of the primitive streak in surface views somewhat before Gerlach; 



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

 Akad. IViss. IV. Abth. 1879. 



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