180 F. M. BALFOUR AND F. DEIGHTON. 



tive streak derived from the epiblast is often obscure, the two sets 

 of cells becoming partially intermingled. The facility with which 

 the cells we have just spoken of can be recognised varies more- 

 over greatly in different instances. In some cases they are very 

 obvious (Ser. c), while in other cases they can only be dis- 

 tinguished by a careful examination of good sections. 



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 ori- 

 ginates 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 shown in Ser. b, 3 ; Ser. c, 2 ; and Ser. 

 Ej 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. XV, 

 fig. I-) 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. 

 Prom 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 ^ 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 



' "Beitr. z. Kenntniss d. Hiilmerkeinis im Beginne d. Bebriituug," 

 ' Sitz. d. k. Akad. Wiss.,' iv Abtb., 1S79. 



