14() University of California Publications. [zoology 



giveu off at the apices. This is contrary to the observations of 

 Harmer who finds that the primary embryo of Crista ramosa 

 bads only at the distal extremity. Calvet COO) also represents 

 the same condition for Grisia denticulata. 



The buds of the primary embryo, from whatever portions of 

 the body they arise, constitute the secondary embryos and from 

 them the free swimming larva' develop. When first set free the 

 secondary embryos of Crista eburnea consist of a small number 

 of cells united into a solid ball, and varying in size from 25 p to 

 35 /* in diameter, containing approximately from 55 to 65 cells. 

 Redivision of the secondary embryos has not been observed in this 

 species. In Grisia ocddentalis, however, there occurs not only 

 the formation of secondary embryos by budding, characteristic of 

 C. eburnea, but also, in some cases, a redivision of these to form 

 tertiary embryos. In these cases the primary embryo breaks up 

 into large masses of cells, the secondary embryos, which in turn, 

 become budding centres, from which tertiary embryos arise, these 

 ultimately becoming the ciliated larvae. This is illustrated in 

 PI. XV, Fig. 29, which represents a section of an almost adult 

 ovicell of C. ocddentalis. On examination of the series of sec- 

 tions to which this figure belongs, it is seen that the ovicell con- 

 tains a few fully developed larva' (lar.). The presence of these 

 indicates that the primary embryo had budded off a few sec- 

 ondary embryos at an early period, and that, later, it divided 

 almost simultaneously into a number of embryos. Some of 

 these may have undergone no further division, while others 

 notably the masses a and b, divided into tertiary embryos. The 

 method of division in these cases is different from that which 

 takes place in G. eburnea, although the result is the same. At the 

 point where the division is about to occur, the nuclei arrange 

 themselves into two linear series parallel to each other, or almost 

 so. In this way two or more masses are formed which round 

 up, separate from each other, and become the tertiary embryos. 

 Many instances of this method of division are shown in the 

 series of which Fig. 29 is a section. In the mass of cells, x, such 

 a process is taking place. PI. XV, Fig. 30 represents an embry- 

 onic mass, taken from another ovicell, showing two tertiary 

 embryos (ter. emb.) which are forming from a large secondary 



