O-V THE CTENOPHORE EGG. 



335 



The first micromere (c t ) of the end-cell is smaller than that of 

 the middle-cell (nij) (Fig. I, i, 2 and 3). To a certain extent the size 

 of the micromerc depends upon that of the tnacromere, it is true, but 

 that is not always the case as will be seen later on. 



Some points of interest regarding the cleavage may be men- 

 tioned. As ZlEGLER pointed out m 11 =mj 2 , while e, , > c 12 . e U-1> the 

 largest of the E micromeres, is divided vertically a little later than 

 its fellows, e 21 , e 3 and nij are also divided vertically (Fig. I. 6). M 

 (basal cells) cleaves into two at no constant period, sometimes quite 

 early as in Fig. I. 5, sometimes much later. 



At about the 140-150-cell stage some cells go into the interior 

 and hand in hand with this process a great cytolisthesis suddenly 

 takes place— a kaleidoscopic change I might say. This converts the 

 embryo into a typical epibolic gastrula. On account of this change the 

 tracing of individual cells beyond this stage is extremely difficult. 

 Then several small " mesoderm cells " are budded off from the 

 entoderm cells (M). They show a fine spinning activity like the 

 polocytes, all of them being connected by a bridge of fine proto- 

 plasmic threads. 



The early cleavage stages of Beroc forskalii was followed up to 

 the 96-cell stage (Table II). The egg of this species is much smaller 

 than B. ovaia, which makes the study a good deal easier. In the 

 main the mode of cleavage of B. forskalii agrees with that of B. 

 ovata. The only difference is in the rythm of cleavage, c,, divides 

 and e 3 is formed, later than in B. ovaia. 



II. Early Cleavage of Callianira. 



The mode of cleavage of Callianira bialata is somewhat different 

 from that of the preceding two (Table III). In some cases the third 

 cleavage is horizontal instead of vertical, the end-cells taking the 

 position o! the first quartet of the ordinary typt of cleavage. 



