92 J. Thos. Patterson. 



development are measured by the relative quantities of yolk ac- 

 cumulated within the ovum. Thus in the Ctenolabrus egg, which 

 contains the least amount of yolk, invagination occurs around the 

 entire margin of the blastoderm, but stronger at the embryonic pole ; 

 in the Batrachus egg, which contains much more yolk than the 

 preceding, there occurs only a slight thickening about the greater 

 part of the margin as ''the initiatory step" in invagination, this 

 thickening soon disappears, and at the embryonic pole alone is there 

 a true germ-ring formed ; and finally, in the Pigeon egg, which is 

 loaded to the extreme with yolk, invagination occurs at the "em- 

 bryonic pole" only, the greater part of the margin lacking even 

 "the initiatory step." 



(2) Middle and Late Gastrulation Stages. 



The entoderm after reaching a stage such as shown in Fig. 32 

 continues to grow forward through the subgerminal cavity as a 

 tongue-like process. At the same time the thinning-out progresses 

 anteriorly and laterally, ordinarily with sufficient rapidity to keep 

 ahead of the advancing entoderm. This results in most blastoderms 

 in the formation of a space just in front of the anterior limit of 

 the entoderm. This space is but a part of the subgerminal cavity 

 that is free from segmentation cells, the latter having passed upward 

 into the differentiating ectoderm. In some few cases, however, no 

 space is found and in such it is impossible to determine the anterior 

 limit of the entoderm. 



The posterior end of a median longitudinal section, in which the 

 length of the invaginated layer equals about one third of the diameter 

 of the blastoderm, is shown in Pig. 34. Only a part (about one- 

 third) of the above mentioned space is included in the photograph. 

 The ectoderm above the space, as well as posterior to it, is not yet 

 differentiated into a single layer, but here and there the lower 

 segmentation cells are seen apparently crowding in between the 

 upper ones. A group of such cells is shown at s. The dorsal lip 

 of the blastopore is much thicker than in Pig. 32, and the method 

 by which it increases will be discussed in another connection. It 



