98 J. Thos. Patterson. 



vestigators (Hacckel, Balfour, Goette, and others), who have main- 

 tained that the entire margin of the avian blastoderm is to be re- 

 garded as the blastopore, for the evidence furnished hj my material is 

 conclusively in favor of the view that but a small part of the margin 

 is the. blastoporic region. The rest of the margin (overgrowth region) 

 I regard therefore as a specialized region, rather than as a place 

 where the upper germ-layer bends under to become continuous with 

 the lower layer. 



The three regions, overgrowth, zone of junction, and germ-wall, 

 are all concerned in the spreading of the blastoderm over the yolk. 

 Since the region of overgrowth has no periblastic nuclei either beneath 

 or external to it, its spreading over the yolk can not be due to the 

 addition of cells from the periblast, unless it be indirectly from the 

 zone of junction. However, the fact that its cells are undergoing 

 rapid division makes it almost certain that the spreading of the 

 overgrowth is due to the multiplication of its own cells. This con- 

 clusion is strengthened by the fact that the cells are digesting the 

 underlying yolk as indicated by the fine granular area. 



As the overgrowth travels peripherally over the yolk, it is followed 

 by the zone of junction, which in turn is differentiating, from its 

 inner edge, ectoderm above and genn-wall below (see anterior end 

 of Fig. 35). The first two regions seldom have greater widths than 

 those in this series (cf. Figs 18 and 28), and hence the germ-wall 

 is continually increasing in width. The suligerminal cavity is also 

 increasing in diameter, but at a slower rate, and in this widening of 

 the cavity there are left around its margin cells which were previously 

 embedded in the yolk. These cells (Fig. XI, I') constitute the under 

 loose layer of the area opaca, and later enter into the formation 

 of the yolk-sac entoderm, and, according to Ruckert, '06, also con- 

 tribute to the vascular tissues. The inner edge of this lower layer 

 becomes united to the free margin of the invaginated entoderm, when 

 the latter spreads over the subgerminal cavity sufficiently to meet it. 

 The first place for this union to occur is necessarily at the postero- 

 lateral regions, and the last place is at the anterior end of the cavity. 



Sections taken slightly to either side of the median line are of 

 interest, in that they have vacuoles or cavities in the dorsal lip 



