BY WILLIAM A. HASWELL, M.A., D.SC. 585 



rapid extension of the blastoderm in all directions. In fig. 3 the 

 two halves of this border have come together to form the axis of 

 the primitive streak. This stage, in which the lateral halves of 

 the primitive streak, meeting along the middle line in a sort of 

 suture, run from the posterior border of the area pellucida to that 

 of the area opaca, has not been observed, and possibly does not 

 occur in the ontogeny of any bird. In figure 4 the area pellu- 

 cida is represented as beginning to send backwards a narrow pro- 

 longation, on the surface of which the primitive streak becomes 

 revealed. The posterior part of the suture, i.e., that part which 

 traverses the area opaca, is not represented in the emu, so far as 

 I have been able to ascertain, even by a posterior notch such as 

 is not rare in the fowl* ; the primitive streak would appear in 

 fact (in the history of the individual) to be formed on the surface 

 of the area pellucida as the latter extends backwards, and to be 

 only foreshadowed in the area opaca by a median thickening of 

 the upper layer, which does not extend far back. The re- 

 maining two figures are intended to illustrate the manner in 

 which, as pointed out by Duval, the anterior end of the primitive 

 streak comes in its later stages to be situated so far forwards 

 simply by the considerable extension of the area pellucida on 

 all sides. 



The ' head-process,' to which repeated allusion has already been 

 made, has been, as regards its relations in the chick, the subject 

 of some discussion. By Kollikerf it is described as being a 

 prolongation forsvards from the anterior end of the primitive 

 streak ; and, in accordance with his view of the origin of the primi- 

 tive streak, he regards it as derived from the epiblast ; he is of 

 opinion that it probably gives rise to the whole of the head. 



GerlachJ describes it as a thickening of the endoderm, and 

 as separated from the cells of the primitive streak behind by an 



* Whitman describes (XXXII) an abnormal blastoderm of the chick in 

 which this line of coalescence is represented on the area opaca by a con- 

 tinuation backwards of the primitive groove to the posterior border. 



+ XXIV., p. 107. 



J XVI., p. 45. 



