I02 CONK LIN. [Vol. XIII. 



axes, the other midway between these axes. The former is the 

 well-developed cross of the gasteropods, but is not prominent 

 in annelids (the intermediate girdle cells of Nereis) ; the latter 

 is the conspicuous cross of the annelids, but the relatively incon- 

 spicuous rosette series of the gasteropods. We have to do, 

 therefore, with two totally different crosses — different in 

 position, structure, origin, and destiny. Wilson says (p. 443): 

 " There is every reason to believe that the annelidan and mol- 

 luscan crosses are analogous, but not homologous structures, 

 whose origin is in some way connected with the mechanical 

 conditions of cleavage. What these conditions are I am unable 

 to conjecture." However, if my interpretation of these struc- 

 tures is correct, the crosses as heretofore defined are not even 

 analogous, and ought not to be compared at all. 



In conclusion then, the cross observed by Wilson in Nereis 

 is not to be compared with the moUuscan cross, but rather with 

 the rosette series of Crepidula, and conversely the molluscan 

 cross is to be compared with the intermediate girdle cells of 

 Nereis, and not with the cross in that animal. To call things 

 which are not to be compared by the same name, and things 

 which are to be compared by different names, would certainly 

 be confusing, and the word cross ought therefore to be changed 

 in either the annelid or the mollusk. Since this word was first 

 used in describing the radiating structure in the molluscan ^g^, 

 and since it has been found in several genera and species of 

 gasteropods, and is, moreover, such a definite structure that its 

 history can be followed very far through the embryology, I shall 

 retain the word cross as heretofore used in the case of mollus- 

 can eggs, and shall take the liberty of changing the designation 

 of the so-called " annelidan cross," calling it the rosette series. 



The cross, as thus defined, is present in the annelid, but is 

 not a prominent structure, as it is in the case of the gastero- 

 pods; on the other hand, the rosette series is present in Crepid- 

 ula as well as in Nereis, but it divides more slowly than in the 

 annelid, and is therefore composed of fewer cells. ^ 



^ In early stages of the cleavage of Ishnochiton, Heath finds that the molluscan 

 cross is typically developed and is very prominent, while in later stages it becomes 

 less marked and the rosette series (annelidan cross) becomes as fully developed 

 and almost as prominent as it is in Nereis. 



