No. 3-] 



PODARKE OBSCURA VERRILL. 



465 



many cases at least, carried by them. Whether they divide to- 

 form eight cells, or remain four in number, does not affect the 

 homology, which in this case is probably complete. 



The other cells of the upper hemisphere have undoubtedly 

 the same general fate, but no comparison can with our present 

 knowledge be made between them. The mucous glands in 

 Amphitrite have the same cell origin as the large glands in 

 Nereis, but the comparison does not extend farther. If we 

 believe in ancestral rudiments in cleavage, we may maintain 

 that the small cell on each dorsal arm of Podarke, which corre- 

 sponds in position to this gland, is really a rudiment of it 

 become functionlesss in the latter genus. It certainly does 

 not give rise to a gland, but I believe that it afterwards 

 enlarges and enters into the general ectoderm of the head. 

 The cross cells differ from their first formation in the number 

 and arrangement of cells, so that no comparison can be 

 instituted. 



2. The TrocJioblasts. — Mead ^ has considered the precise 

 similarity between the formation of these cells in Amphitrite 

 and Clymenella, the similarity extending not only to the pri- 

 mary but to the secondary trochoblasts. In Arenicola, also, 

 the prototroch is formed as in Amphitrite, and thus far the 

 homology holds. As stated above, however, the Podarke pro- 

 totroch arises in a different way. The comparison can best be 

 seen from the table. 



Primary Trochoblasts. 



Amphitrite 'j 

 Arenicola ^ 

 Clymenella J 



Podarke. 



lao.i.i 



I a 2.1.2 etc., in all 

 I a 2.2.1 four quadrants. 

 I a 2.2.2 



The same as the others. 



Secon 



f 2a I.I 

 ■{ 2a I.I 

 [ 2a 1.2 



r I a 1.2 

 ^ 2a I.I 

 1^ 2a 



DARv Trochoblasts. 



in A, B, and C 



quadrants. 



I 



in A, B, and C 

 quadrants. 



In the description of the trochoblasts, I have distinguished 

 between "tertiary" and "secondary." For convenience both 

 are here grouped under " secondary." 



Thus, in Podarke, three out of the twenty-five cells which 

 make up the completed prototroch are different from those 



1 See Appendix to Literature. 



