96 J. Thos. Patterson. 



under surface of the differentiating ectoderm. That part of the 

 subgerminal cavity lying beneath the entoderm (E) is to be regarded 

 as the archenteron, which communicates with the blastopore by a 

 narrow passage situated just below the dorsal lip (D). At its 

 union with the deeper cells of the lip, the entoderm is very thick, 

 but gradually thins out anteriorly, ending with a thin irregular 

 margin slightly beyond the center of the blastoderm. 



The changes described above can be made clearer by a study of 

 longitudinal sections. Thus in the photograph of a median section 

 (Fig. 35) the various regions are easily recognized, and in the en- 

 larged drawing of the posterior end (Fig. 19) the dorsal lip is seen 

 to be composed of compact cells, all of which are completely delimited 

 by cell-walls. Directly above the lip there is no distinct ectoderm, 

 but anterior to the point u it is well differentiated and in only a few 

 places (s) are lower segmentation cells crowding upward into it. 

 The entoderm at its union with the deeper cells of the lip is five cells 

 thick, but anteriorly gradually decreases, finally ending with a free 

 margin (Fig. 35, e — to the left). At this stage the entoderm can 

 not be said to be a distinct layer, for its cells are arranged more or 

 less into groups. In the archenteric cavity (ac), which lies between 

 the entoderm and the yolk, are several large yolk masses, some of 

 which are in the act of rising from the floor. Posteriorly the archen- 

 teron communicates by a narrow passage with the blastopore. 



The conditions presented in this section very much resemble those 

 of a corresponding section from a teleostean blastoderm in which 

 invagination is well advanced. Thus Miss Wallace's ('99) Fig.5, 

 PI. Ill, not only compares very favorably with Fig. 19 in the ap- 

 pearance of the dorsal lip, but also as regards the method by which 

 the entoderm grows forward; for I consider it better to" describe 

 the entoderm as now advancing anteriorly by a multiplication of 

 its cells and their gradual arrangement into a single layer. This 

 view is in accord with the account for Ctenolabrus as given by 

 Agassiz and Whitman ('84). At this stage the main difference 

 between the teleostean and pigeon blastoderms is that in the former 

 the ectoderm is from three to five cells thick at the embryonic pole, 

 while in the latter it is but one cell thick (Fig. 37). This difference 



