PLATE XXXIX. 



Fig. 1. Cteuolabru.s, under (inner) surface. Treated as Figs. 1 and 2, [Plate XXXVIII], four 



(3) hours after 32-cell stage. One hour after Figs. 1 and 2 [Plate XXXVIII]. In this cap the 



nuclei of the eiidoderm are strongly colored and very well defined. One can see by the nuclei 



that the endoderm extends for only a short distance under the cap. The majority of the 



nuclei are outside the cap. 



Fig. 2. [Seven .sections]. Three hours after 32-oell stage, [be = blastocoele; pb = periblast]. 



(89) (Os, Mk. .3ds). 



Second section shows only one or two outlined cells, the outlines being indistinctly marked. 



The third section shows two well-outlined cells; the right is sharply marked inferiorly. 

 There is a partial outline of a dark cell of the cap. 



The fourth section shows the first appearance of the sub-germinal plate. All the rod cells 

 are shaded by dotting. 



The sixth section shows the subgerminal plate thinner. 



The sixteenth section shows on the left a red cell that looks as if it was to enter into cap. 



The twenty-sixth section is near middle. Here sometimes no nucleus is seen in periblast, 

 at other times one or even two. Subgerminal plate very thin, wavy in outlines as in section 

 sixteen. 



The twenty-eighth section shows an inner cell in process of splitting off, possibly destined to 

 become one of the cap cells. 



The thirty-third section showed a similar case. 



I think it is possible that cells are still added to the cap, but that this process is nearly con- 

 cluded, so that the periblast as a cell layer may be now considered established. 

 Fig. 3. [Later stage. 6f^ = blastocoele; cp = epidermis; p6 = periblast]. Section near middle. 



(90) The periblast is very thin, vanishing or nearly so near the center of field. Epidermis well 

 marked off. 



On left is a single periblast cell that looks as if it was a cap-cell. The fact that the periblast 

 becomes so very thin indicates that cells are added to the cap up to about the time the ring 

 begins to form. 



The nuclei of the ijeriblast are still confined to the thickening beneath the margin of cap; 

 on the left [i. e. lower end of figure] a single nucleus is somewhat advanced from the margin 

 towards the center, but this is exceptional in these sections. 



