CLEAVAGE IN ANNELIDS. 97 



of each cell and a rotation of the upper portion in a clockwise 

 direction. The next division to the sixteen-cell stage is anti- 

 clockwise ; so that we can speak of an alternating direction of 

 cleavage as characteristic. This alternating direction is often 

 obscured in cases where the segmentation does not follow a 

 regular geometrical progression ; e.g., Nereis, Crepidula, and 

 Unio, and beyond the sixty-four-cell stage it is lost more or 

 less rapidly in all. Why cleavages should always take such 

 definite directions we do not know. Mechanical conditions 

 undoubtedly play a large part in the arrangement, but mechan- 

 ical explanations are not sufficient to account for the definite 

 direction assumed by each cleavage. The fact that in molluscs 

 with a dextral shell the first rotation is to the right, while in 

 those with a sinistral shell it turns to the left, may indicate 

 some relation between the form of the first divisions and the 

 symmetry of the future body ; but in the present state of our 

 knowledge such an assumption would be premature. 



The important points in the development thus far are these : 

 I. At the sixteen-cell stage a separation of certain cells form 

 the first generation of ectomeres, which go to form the primary 

 prototroch, and are, therefore, trochoblasts. These divide twice, 

 and twice only; so that in the sixty-four-cell stage there are 

 sixteen of these cells, which become ciliated and form the larval 

 locomotor apparatus. 2. The second micromere, separated from 

 the large macromere D, is larger than any of the other members 

 of the same quartette, and, as further study shows, forms a 

 large part of the ectoderm of the future trunk of the animal. 

 According to Dr. Mead, all of the ectoderm of the body behind 

 the first septum arises from this cell. 3. The separation of the 

 mesoderm, in large part at least, in the fourth micromere in 

 quadrant D. 4. The separation of four rosette cells in the 

 upper pole, which later divide once, and carry the apical tuft of 

 cilia, which is such a marked feature of the larva. 5. The 

 segregation of the entoderm in the macromeres, which invagi- 

 nate to form the alimentary canal. 



Evidently, then, the egg of such an animal as AmpJiitrite 

 possesses from the beginning a definite orientation. This is 

 indicated by the "polar furrow/' which always has a definite 



