46 CONKLIN — EMBRYOLOGY OF A BRACHIOPOD. [April 4, 



angles (Figs. 5 and 7). In the eight-cell stage a considerable 

 cleavage cavity appears with openings at the animal and vegetal 

 poles (Figs. 6 and 7). 



The cleavage forms just described and represented in Figs. 1-7 

 are among the most regular ones observed ; others are irregular and 

 unequal from the first. One of the most common variations is pro- 

 duced by very unequal divisions, the chief mass of the egg seeming 

 to bud off small cells, usually at the animal pole (Fig. 8). Later 

 stages in which one or two of the blastomeres are much larger than 

 the others, and in which the cleavage cavity is smaller than usual, 

 are not infrequently found. Such eggs appear to give rise to nor- 

 mal blastulae, in which all the cells are of equal size, by the more 

 rapid division of these larger blastomeres. 



The eight-cell stage gives rise to the sixteen-cell stage by the 

 meridional division of each of its blastomeres. Fig. 7 shows each 

 blastomere of the eight-cell stage indented at its periphery, pre- 

 paratory to this division, which occurs simultaneously in all of the 

 cells. The sixteen cells shown in Fig. 9 and in optical section in 

 Fig. 10 are all of approximately the same size. Except for the 

 occasional presence of the polar bodies at this stage it would be 

 impossible to distinguish the animal from the vegetal pole. The 

 cleavage cavity is now larger and it no longer opens widely to the 

 exterior. 



In subsequent cleavage stages division does not take place simul- 

 taneously in all of the cells ; this is shown, for example, by Fig. 1 1, in 

 which twenty cells are present, some of which are considerably 

 larger than the others. In the eggs represented in Fig. 1 2 about 

 forty-eight cells are present and some of these are larger than 

 others, indicating that with them division has been delayed. The 

 egg shown in Fig. 12 has been flattened by the cover glass, so that 

 its apparent diameter is greater than normal ; at the same time the 

 blastomeres are separated from one another in an abnormal manner. 

 At all stages the blastomeres are but loosely joined together and they 

 break apart at the slightest pressure. In the later development I 

 have found many embryos which are about one-half or one-quarter 

 the size of the normal embryo, and it seems likely that such 

 embryos have developed from isolated blastomeres of the two- or 

 four-cell stage. 



After this brief description of the cleavage, I think it will be 

 quite apparent to everyone that it would be extremely difficult, if 



