214 SIDNEY F. HAEMER. 



It is presumably at this stage that fertilisation takes place ; 

 but I have in vain looked for any evidence of perforation in 

 the terminal membrane of the ovicell, or for traces of sperma- 

 tozoa inside the tentacle-sheath. This fact is not really sur- 

 prising when it is remembered that the finer details of the 

 highly calcified ovicell of Crisia can hardly be examined 

 except by means of sections ; and that the spermatozoa are 

 very minute. 



In fig. 5 the whole ovicell has considerably increased in 

 length. Its irregular form is of course due to shrinkage 

 caused by the action of reagents. The ectocyst is not repre- 

 sented in the figure. The ovicell was probably at about the 

 stage represented in fig. 21. 



The valve (c/. pi. xii, fig. 10, of my former paper) is now 

 developed as a fold of the ectoderm on the back wall of the 

 ovicell. The aperture has no longer any obvious opening to 

 the exterior ; and the tentacle-sheath has increased in size, its 

 walls having become very thin, except at its distal end, which 

 is considerably thickened. 



The follicle is slightly larger than before, and its nuclei have 

 obviously increased in number. In place of the egg found in 

 the preceding stage, there are now three egg-like cells, which 

 are not in contact with one another ; and which I regard as 

 blastomeres. Remains of the follicle-cavity are still present. 



Although I have no direct evidence that the ^^ blastomeres ^^ 

 are really derived from the egg, their subsequent history leaves 

 room for little doubt on this point. The details of the forma- 

 tion of the primary embryo in Crisia remind one strangely 

 of the early development of Sal pa, as described by Salensky 

 (28). This is true not merely of the segmentation of the 

 ovum, but also of the later relations of the embryo to its 

 follicle. Salensky states, for instance, that the blastomeres of 

 Sal pa may at first be entirely disconnected from one another 

 (I. c. pi. X, fig. 10 ; pi. xxii, figs. 3, 1). 



In the next stages, of which I have numerous preparations, 

 but which I have not figured, the number of blastomeres gra- 

 dually increases. I have been unable to make out any regu- 



