APPENDIX. 



4S7 



I will first briefly describe the facts common to the three 

 forms and to Nereis, and afterwards point out some of the 

 specific peculiarities. 



The first five cleavages (to the thirty-two-celled stage) are 

 nearly identical in all, excepting slight differences in the relative 

 size of the blastomeres. Three sets of ectomeres are formed in 

 the typical manner, and the somatoblasts, X and M, have essen- 

 tially the same origin, position, and relation to the blastopore- 

 margin. In all, the apical rosette and the cross arise in precisely 

 the same manner, and at nearly the same period ; and in all the 

 first bilateral divisions first take place after the reduction of the 

 left posterior macromere, D, to the size of its fellow, C. In all, 

 furthermore, the primary girdle, consisting at first of eight and 

 afterwards of twelve cells, arises exactly as in Nereis. The pro- 

 totroch is developed in the region of the girdle-cells, but I have 

 not yet succeeded in tracing its origin in detail. In all, finally, 

 the first somatoblast, X, undergoes three unequal divisions, as 

 in Nereis, thus giving rise successively to x^ on the right side, 

 x^ on the left, and x^ in the middle dorsal line. It then divides 

 into equal parts, X, X, the posterior proteloblasts. 



II. 



I pass now to the leading individual peculiarities, some of 

 which are very interesting from a comparative point of view. 



(i) Spio differs from the others in the immense relative size 

 of the left posterior macromere, D, and of the somatoblasts, X 

 and M, derived from it. 



(2) In Spio, Aricia fcetida and Polymnia x^ and x"^ are very 

 much larger than in Nereis, so that after their formation X 

 becomes considerably reduced in size, and the posterior protelo- 

 blasts are relatively small. 



(3) The later development of the cross in the same three 

 forms differs somewhat from that of Nereis, the cells c^-^, d^'^ 

 (" nephroblasts " in Nereis) being much smaller and formed in a 

 slightly different position. 



(4) Polymjiia differs from the other forms in the fact that 

 the second somatoblast, M, is from the first partially covered by 

 the macromere, D, and at the time of its fission extends to the 



