136 University of California Publications. . [zoology. 



a similar position in the two-cell stage. The separation of the 

 blastomeres and the interpolation of small cells is a character- 

 istic of the early stages of Crista, and in most older stages than 

 the two-cell stage the blastomeres divide cpiite independently 

 of one another. PL XIV, Fig. 22, represents a four-cell 

 stage in which again are shown the separation of the blastomeres 

 and the interpolation of the follicle cells (sm.fl.cls.). This 

 ovicell is further interesting as showing the characteristics 

 of the follicle cells. These now surround the embryo so 

 that it lies in the center of a sphere consisting of a number 

 of concentric layers composed of cells which form a net -work 

 by the uuiou of their protoplasmic processes (fl. els.). In the 

 interior of the spherical follicle the four blastomeres of the 

 embryo may be distinguished by their larger size (hi.). The 

 other cells of the interior (sm.fl. els.) are of various sizes, those 

 nearest the embryo being the smaller, those nearest the inner 

 layer of the follicle, the larger. An examination of a large 

 number of specimens shows that the multiplication of the small 

 cells is accompanied by a diminution in number of the cells of 

 the concentric layers. The former seem without doubt to be 

 derived from the latter and to represent a stage in their absorption. 

 PI. XIV, Fig. 23, represents an embryo in the eight-cell stage, 

 only four blastomeres being visible in this section. The sepai-ation 

 of the cells of the embryo is clearly brought out, the blastomeres 

 being perfectly distinguishable by their larger size and their 

 different staining capacity. The increase in the number of small 

 interior cells is noticeable as is also the decrease in the follicle 

 inclosing the embryo. 



This separation of the blastomeres continues to be a striking 

 feature of the embryonic development of Crista until about the 

 twenty or twenty-four cell stage when the blastomeres unite to 

 form a more or less compact ball. Harmer ('93, '97 and '98) 

 has shown that it is characteristic of this and also of other 

 genera of the Cyclostomata viz., Lichenopora and Tubulipora. 

 In a recent paper, Braem reports a somewhat similar method 

 of cleavage for Plumatella. According to this writer the 

 egg of Plumatella consists of two quite distinct parts, an outer 

 granular zone, and an inner zone containing the nucleus. 



