318 GILMAN A. DREW. 



where the polar body will appear. In the preparation for 

 the first cleavage a similar swelling is formed on the side 

 opposite the polar bodies. When the egg divides, the divid- 

 ing wall passes to one side of this swelling. The two blasto- 

 meres are accordingly rather unequal in size. The difference 

 in the size of the two blastomeres seems to depend upon the 

 size of the swelling that precedes their formation. Cleavage 

 into four and eight cells (Text-figs. C and D) are typical. 

 The polar bodies retain their position on the animal pole 

 until the embryo acquires cilia, when they are rolled around 

 on the inside of the membrane. No attempt has been made 

 to follow out the fate of the individual cells. 



In the sixteen-celled stage, figs. 2 and 3, a small cleavage 

 cavity is present. Later this becomes slightly more pro- 

 nounced. The cells on one side of the blastula divide more 

 rapidly than those on the other side, and push over them in 

 the form of a cap (fig. 4). A pocket appears between the 

 large cells at such a point as is indicated by the asterisk in 

 fig. 4. Just hoAv this pocket is formed is still a matter of 

 some doubt, but it seems to be formed by the separation and 

 division of some of the larger cells. This pocket (fig. 8) can 

 now be compared with the invaginate portion of a gastrula. 

 It represents the first appearance of the gut. 



About the time that the pocket is formed most of the 

 smaller surface cells acquire cilia (fig. 6), and the embryo 

 begins to roll around in the membrane. The cilia are all 

 short, similar in appearance, and seem to be evenly scattered 

 over the surfaces of the cells. In whole mounts the 

 boundaries of the surface cells are not very distinct, but the 

 cells do not seem to have a very definite arrangement. 



From these small surface cell§, that at this stage appear 

 very much alike, the test,^ the apical platC; and the cerebral 

 ganglia are formed. 



The large cells near the blastopore do not bear cilia, at 



^ I use the term " test " here, as in former publications, to designate the 

 surface cells that bear cilia and may be homologised with the velum of other 

 forms. 



