268 J. THOS. PATTERSON. 



not reach the anterior limit of the subgerminal cavity until from 

 two to four hours after laying, at which time the mass disappears. 



Origin of the Primitive Streak. 



The primitive streak in the pigeon's blastoderm becomes vis- 

 ible in surface views between the fourth and fifth hours of incuba- 

 tion. In sections, however, it can be detected as early as two 

 hours previous to this time. Its first appearance in section is 

 that of small protuberances of cells on the under surface of the 

 ectoderm situated along the median line. In their longitudinal 

 extension these swellings reach from the posterior edge of the 

 area pellucida to a point lying about half way between this edge 

 and the center of the blastoderm (Fig. 1 5, ps). Under high mag- 

 nification (Fig. 1 7, ps) these swellings are seen to be groups of 

 rapidly dividing cells, which at first are separated from the gut- 

 entoderm, but upon further growth come in contact with it. At 

 the stage represented in Fig. 17 the gut-entoderm is a single 

 layer of flattened cells and is directly continuous with the yolk- 

 sac entoderm (Figs. 15-17, Y). The latter is thicker just poste- 

 rior to the primitive streak than in any other region of the yolk 

 zone. 



The evidence afforded by a study of gastrulation indicates the 

 line along which one must look for an explanation of the origin 

 of the primitive streak. During the progress of concrescence 

 there are laid down along the sides of the future longitudinal axis 

 of the embryo strips of primary ectoderm, which are fused along 

 the median line. These strips were previously the superficial 

 cells of the right and left halves of the dorsal lip of the blasto- 

 pore. For four or five hours after the closing of the blastopore, 

 this median strip cannot be distinguished from the adjoining ecto- 

 derm. It is not until the protuberances begin to make their ap- 

 pearance that any difference can be seen, and even then, the double 

 structure of the median region is not evident. In fact, it is only 

 when the primitive groove appears that this bilateral structure be- 

 comes clear. 



In conclusion, I may say, that during the process of gastrula- 

 tion only the gut-entoderm is involuted ; the chorda and meso- 

 derm arise from the primitive streak, which represents the fused 



