20 SIDNEY F. HARMBR. 



consequently, appreciably smaller than that in C. ramosa, 

 although in old ovicells of the latter species even, the quantity 

 of the protoplasmic reticulum may be considerably reduced, 

 by the development of the larvse at its expense. The primary 

 embryo, on the contrary, is relatively very large. It has 

 differentiated a distinct external layer of nuclei, which will 

 give rise to the external layer of the secondary embryos. 

 Throughout the development, the budding secondary embryo 

 of C. eburnea differs considerably from that of C. ramosa; 

 although the fundamental facts are the same in both species. 



I have, unfortunately, no satisfactory sections of the ovicells 

 of other species, which I found more difficult to obtain than 

 the two former species. It is necessary to have a large stock 

 of material in order to study the development ; as it usually 

 happens that a very small proportion of the colonies found are 

 provided with ovicells. 



The stages intermediate between figs. 9 and 11 have not 

 been figured ; but it is easy to describe their general develop- 

 ment. After the stage shown in fig. 9 (but not until then) the 

 embryo increases in size, and rapidly transforms itself into the 

 characteristic mass of embryonic cells from which the young 

 larvae are budded off. 



The history of the aperture of the ovicell has, so far, not 

 been considered in sufficient detail. Its commencement as an 

 invagination of the endocyst has been seen in fig. 3. When 

 the egg has begun to segment (fig, 5), the opening of the 

 invagination has closed. The distal end of the tentacle-sheath 

 is, however, now thickened ; and the valve is commencing to 

 develop. 



In fig. 8 (C. eburnea), the valve is practically complete; 

 the distal thickening of the tentacle-sheath has increased, but 

 the invagination constituting the primary aperture has not 

 materially altered. The ovicell is completely calcified except 

 in the region of its aperture, which is beginning to grow out 

 into its tubular form. This part is covered merely by uncal- 

 cified ectocyst. 



Fig. 12 represents a slightly earlier stage in C. ramosa. 



