120 J. Thos. Patterson. 



ing upward of the lower segmentation cells in l^etween the superficial 

 ones, finally reducing the entire central region to a single layer — the 

 primary ectoderm. The thinning-out begins slightly posterior to 

 the center of the disc and then spreads in all directions, but with more 

 rapidity toward the posterior margin. The thinned-out central region 

 is the beginning of the area pellucida. 



2. Between thirty and thirty-three hours after fertilization the 

 zone of junction, or the region where the marginal cells are open to 

 the white yolk or periblast, becomes interrupted for a distance of 

 seventy to eighty degrees at the posterior margin. Hence, this mar- 

 gin of the blastoderm now ends with a free edge. The interruption 

 is but the separation of the blastodisc from the underlying periblast, 

 and has associated with it the degeneration of periblastic nuclei. 



3. At about thirty-four hours after fertilization (or seven hours 

 before the egg is laid) there occurs the gastrula-invagination. This 

 consists in the rolling under of the free posterior edge of the blasto- 

 derm, together with the simultaneous fonvard growth of the involuted 

 cells. The invaginated cells are arranged in the form of a tongue- 

 like process, which finally penetrates the subgerminal cavity (enlarged 

 segmentation cavity). It does not reach the anterior limit of this 

 cavity until from three to four hours after the beginning of incu- 

 bation. 



4. Immediately after the gastrula-invagination occurs, the rounded 

 posterior margin thickens up ; in part by the multiplication of the 

 cells in situ, but mainly by the movement of material from the right 

 and left halves of the dorsal lip, which come together and coalesce 

 in the middle line — a process to be regarded as a form of "con- 

 crescence." 



5. The median region formed by the coalescence of the lips of 

 the blastopore is the primordium out of which the primitive streak 

 develops. Since the primitive streak gives rise to the mesoderm and 

 chorda, its formation is to be considered as a part of gastrulation. 



University of Chicago, June, 1908. 



