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J. THOS. PATTERSON. 



blastopore. All this will become clear upon examining a blas- 

 toderm during the period in which gastrulation is at its height, 

 and comparing it with a stage such as is shown in Fig. 5. 



Fig. 6 is a reconstruction of a blastoderm at the height of 

 gastrulation, which was taken five hours before laying, or thirty- 

 six hours after fertilization. It represents a dorsal view as though 

 the ectoderm were transparent. On the anterior and lateral mar- 

 gins is a clear crescent-shaped area (0), the region of over-growth. 



D F H 



FlG. 6. A diagrammatic reconstruction of a blastoderm taken thirty-six hours 

 after fertilization, or five hours before laying. It represents the ectoderm as trans- 

 parent. O, region of overgrowth ; Z, zone of junction ; Y, yolk zone ; PA, outer 

 boundary of the area pellucila ; S, beginning of yolk-sac entoderm ; E, region cov- 

 ered by invaginated or gut-entoderm ; R, dorsal lip of the blastopore. Lines drawn 

 through CD, EF, and GH represent the planes of the sections illustrated in Figs. 

 8, 9 and io, respectively. The arrows at posterior margin indicate the direction of 

 movement of the halves of the margin. X 2 7- 2 - 



Between this area and the subgerminal cavity (PA) is the germ- 

 wall, in which two distinct zones can be recognized. The outer 

 of these (Z) may be designated as the zone of junction, 1 and is 

 1 This term was first used by Agassiz and Whitman, '84. 



