vol.3.] Bobertson. — Embryonic Fission in Crisia. 135 



whole cell to that of the nucleus is 2:1.5 or less, whereas in 

 the blastomeres it is i! : 1 or inure. This latter rule holds in the 

 present case. Thus in Fig. 19, although the blastomere is 

 as large as many ova, i.e., 14.4 i>- in diameter, its nucleus 

 is only 7.2 a 1 , while in Fig. 20 the blastomere measures 9 /* 

 with a nucleus of 3.6 /*. in the first the ratio is just 2:1, in 

 the second it is slightly greater. Additional evidence that 

 these bodies are not ova is afforded by the difference in 

 their rate of growth since cleavage. In a second instance an ovicell 

 in the same stage contained an embryo of two blastomeres still 

 adhering to each other as if division had but recently occurred. 

 The cells of this embryo are relatively very small, the two meas- 

 uring 14.2 /*, about as much as a single ovum. The cells of 

 the bud have much the same appearance and bear the same relation 

 to the embryo as those shown in the bud of Figs. 1!' and 20. 

 That the latter represent an early stage in the development of 

 the embryo is further shown by the fact that the blastomeres are 

 not yet surrounded by the cells of the bud (pi], bd.) . Neverthe- 

 less that some time has elapsed since cleavage occurred is showu 

 again by the presence of the small cells between the blastomeres. 

 Furthermore, the separation of the blastomeres shows that cell 

 division takes place some time previous to or following very close 

 upon the formation of the ovicell. In this ovicell there is yet no 



appearan f the tentacle sheath, the two lines of cells extending 



downward from the anterior border being those that form the 

 vestibule (vest. ) . 



A somewhat later stage of embryonic development is repre- 

 sented in PI. XIV, Fig. 21. Here the embryo (< ml*.) contains at 

 least three blastomeres which are not only surrounded by the 

 follicle but are pushed apart and separated by the interior 

 cells. The beginning of the tentacle sheath is shown in the layer 

 of cells separating from the distal surface of the bud, the cavity 

 formed between the outer surface of the bud and this layer (t> nt .) 

 being the cavity of the tentacle sheath [tent. car.). Here again 

 the blastomeres have the same ovum-like appeai-ance as in the 

 two-cell stage, but they are smaller, the larger of them being 

 10.8 p- in diameter, and the other two about 7.2 m. In this 

 stage the cells between the blastomeres are smaller than those in 



