lO WHEELER. [Vol. VIII. 



united with the head of the embryo (Fig. I, A and B) before 

 the stage of Fig. 15 and soon after this stage is, I believe, 

 normally united with it. This union is probably purely 

 mechanical — the organ remaining at its place of origin on 

 the surface of the yolk, while the embryo lengthens till its 

 head unites with the posterior end of the organ. This union 

 is of brief duration as is seen in Fig. 3 (Stage C). 



During this stage the caudal tip of the embryo shows a 

 tendency to bury itself in the yolk. The amnion and serosa,^ 

 hitherto closely applied to each other, now separate at 

 the caudal end, where, as I have said, they first arose as a 

 crescentic fold. Soon the tendency to enter the yolk becomes 

 more pronounced so that the tail curls back and leaves the 

 ventral face of the q,^%. Meanwhile the remainder of the 

 embryo moves down the ventral face a short distance, thus 

 pushing its tail still further into the yolk and causing the 

 separation of the envelopes to advance still further headwards. 

 The indusium does not accompany the embryo in this move- 

 ment, but remains nearly or quite stationary ; consequently 

 the head gradually separates from the organ till it is connected 

 only by means of a slender band of cells in the median 

 line. (Fig. 3 and Fig. 16.) This link soon ruptures and the 

 indusium is set adrift from the embryo, or, more precisely, 

 the embryo is set adrift from the indusium. (Fig. 4, Stage D.) 

 In profile the embryo now resembles the small letter j, — the 

 dot being supplied by the isolated indusium. 



Important changes begin to affect the indusium during or 

 more frequently just after its separation from the embryo. 

 The closely packed cells at the periphery, as indicated by their 

 nuclei, begin to arrange themselves radially (Fig. 16). Some 

 of the large nuclei of the serosa may be seen encroaching on 

 the edges of the disk from all sides, leaving only the median 

 portion free. Sections show that the organ is now forming an 

 amnion like that of an embryo. In the middle of the disk 

 appear several shrunken but distinctly defined nuclei which are 

 proved by focusing to be confined to the surface of the organ.^ 



1 Only four of these peculiar bodies are represented in the figure {nii)\ there 

 were several others in the same preparation, but for the sake of clearness I have 

 omitted them in the drawing. 



