No. 2.] DEVELOPMENT OF MARINE ANNELIDS. 293 



B. 



cell of the 8-ceII stage arises by an unequal division, but divides 

 about equally {D^ and d'^, PL X). The divisions corresponding 

 to the latter in the other three quadrants are extremely unequal, 

 though the relative amount of yolk in the cells is less. The 

 first division of the original mesoderm cell in Amphitrite is equal, 

 the division of each product 

 extremely unequal. In Clyme- 

 nella the original mesoderm cell 

 divides equally, and the daugh- 

 ter cells also. 



Cells without much yolk divide 

 equally or unequally without re- 

 gard to size, as may be seen by 

 simply examining the figures of 

 the upper hemisphere in the 

 various eggs at all stages, espe- 

 cially those representing the 

 formation of the rosette, cross, 

 and secondary prototrochal cells 

 in Amphitrite and Cly^nenella. 



As to the influence of gravity in determining the size of the 

 cells, it needs only to be said that many eggs, i.e., Afupkitriie, 

 Clymenella, and Nereis, develop normally whether in one 

 position or another.* 



Similarity in position of cells does not insure similarity in the 

 relative size of products. In the 32-cell stage of Lepidonotus, 

 four cells lie at the animal pole in the same position, with refer- 

 ence to surrounding cells, as the four cells at the vegetative 

 pole. The eight are of about the same size, and divide une- 

 qually in the same direction and at the same time; but at the 

 animal pole the resulting central cells are the smaller products, 

 while at the vegetative pole the central cells are the larger prod- 

 ucts. In Discoccelis the four vegetative cells have the same posi- 

 tion as in Lepidonotus, but the four central products of the next 

 division are the smaller. The division of the paired mesoderm 

 cells and the secondary trochoblasts in Amphitrite and Clyme- 

 nella furnish additional illustrations (text Figs. XXII, XXIII). 



* Dr. W. M. Wheeler tells me that the same is true of the egg of Blatta. 



Fig. XXII. — Shows the relative size of the 

 products of the paired mesoblasts, M aud 

 M, after their first division ; A, A mphi- 

 trite ; B, Clymenella. 



