No. 2.] DEVELOPMENT OF MARINE ANNELIDS. 285 



Aricia, Polyjnnia, Amphitrite, Clymenella, Chcetopterus , and 

 Lepidonotus. Conklin finds a similar rosette in the mollusc 

 Crepidula, and Heymons mentions it in U^nbrella, but gives no 

 figure. Lillie did not follow the cleavage of the apical cells in 

 Unio far enough to find out whether the rosette existed or 

 not. In Amphitrite and Clymenella all four rosette cells divide 

 again in the regular reverse oblique direction, and Wilson figures 

 one of these cells as dividing in this way in Nereis. Their 

 division in other forms has not been recorded. In Lepidonotus 

 it is certain, and in Amphitrite and Nereis extremely probable, 

 that the rosette cells, or their products, bear the cilia of the 

 apical tuft. The rosette cells, therefore, seem to have the same 

 origin and fate. 



The four cells which alternate with the four of the rosette are 

 the parent cells of the cross^ a peculiar pattern first described 

 by Wilson 1 in Nereis, Polymnia, and Aricia. I have found an 

 exactly similar cross in Amphitrite, Clymenella, and Lepidonotus. 

 The cleavage of the four parent cells departs so abruptly from 

 the method of the earlier cleavage, and is so similar in all 

 annelids where the cross occurs, that it deserves special men- 

 tion. The cross has been described in representatives of seven 

 genera belonging to six different families; in Spio, Polymnia, 

 and Aricia without figures, so that we will confine our compari- 

 sons to Nereis, Lepidonotus, Amphitrite, and Clymenella. In 

 all of these the four parent cells arise in the same way and lie 

 symmetrically, two on either side of the middle line, one in each 

 quadrant. Each divides horizontally and bilaterally, so that 

 the pattern on one side is the counterpart of that on the other, 

 and I have searched in vain for any reminiscence of oblique 

 cleavage. The outermost cells in each arm soon divide again 

 in the same direction, making three cells in a row. 



Peculiar interest attaches to the middle cells of the dorsal 

 arms, because their destiny has been ascertained in Nereis and 

 Amphitrite. In Nereis they do not divide again, but sink 

 into the interior of the ^gg and become what Wilson 

 provisionally calls "head-kidneys." In A^nphitrite also they 

 sink beneath the surface, except for a slender stalk, and 

 form a pair of immense unicellular mucous glands. Although 



