94 J. Thos. Patterson. 



older blastoderms the affected region is not found in the entoderm, 

 but in the ectoderm and mesoderm, showing that the involution 

 has ceased, and the further extension of the entoderm is now brought 

 about bj an ingrowth, in which cell division and the centrifugal 

 expansion of the ectoderm play an important role. The latter 

 two processes are doubtless factors in the extension of the entoderm 

 throughout the entire jDeriod of involution, but they are not so con- 

 spicuous during the earlier stages of invagination. 



We must now pass to a series in which gastrulation may be said 

 to have reached its height, and one in which several structures and 

 processes hitherto unnoted must be considered. The reconstruction 

 of this series aj)pcars in Fig. XI. The tongue-like process of ento- 

 derm (-£'), the dorsal lip of the blastopore (-D), the germ-wall {GW), 

 and the zone of junction (z) were considered in connection with 

 Fig. VIII, but they have all undergone important changes. Thus, 

 the germ-wall extends almost around the whole inner margin of 

 the zone of junction, and on the lateral margins its cells extend 

 into the subgerminal cavity, within the edge of the area pellucida. 

 This extension of cells is not due to an ingrowth from the inner 

 edge of the germ-wall, but rather to the spreading of the sub- 

 germinal cavity by the liquefaction and fragmentation of the under- 

 lying yolk. 



The changes in the dorsal lip consist in the growth of the right 

 and left halves toward each other and their simultaneous fusion 

 in the middle line, that is, in the plane of the longitudinal axis of 

 the future embryo. A blastoderm in which the line of fusion was 

 seen in surface view is shown in Fig. V, L. The movement of 

 material is participated in by the more lateral parts of the margin, 

 namely, the horns of the zone of junction, and as they move toward 

 the median line they are at the same time being carried centrifugally 

 by the expansion of the blastoderm, and in this way the fused 

 halves of the lip are gradually being enclosed within the inner edge 

 of the zone. The question of this movement of material will be 

 fully discussed in connection with the description of experiments 

 performed to throw light on the method by which the embryo arises. 



The region of overgrowth (0), which is represented in the figure 



