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J. THOS. PATTERSON. 



completely encircle the rim of the blastoderm, but it is interrupted 

 for a distance of about 70 degrees at the posterior margin. The 

 form of the germ-wall is that of a crescent, with its horns stop- 

 ping slightly short of the area over which a single layer has been 

 developed. 



During the two or three hours immediately following the stage 

 just described, the thinning out continues to extend anteriorly 

 over the central portion of the blastoderm, and at the same time 

 spreads laterally. By the time the posterior third of the blasto- 



Fig. 5- A diagrammatic reconstruction from sections of the blastoderm from 

 which Figs. 3 and 4 were drawn. GW, germ-wall. Numbers I, 2, 3, etc., repre- 

 sent the regions of the blastoderm which are one, two, three, etc., cells deep, respec- 

 tively. The broken line around 1 indicates the region where the blastoderm is ap- 

 proximately one cell deep. X 2 7- 2 



derm is thinned out to approximately one layer there occurs the 

 initial step in gastrulation. Owing to the individual variation in 

 the development of eggs, some difficulty has been experienced 

 in securing a complete series through this brief period, so that at 

 present I am not in a position to state positively in what this 

 initiatory step consists. The evidence, however, inclines me to 

 believe that it is brought about by a rolling under of the thin 



