l66 CONKLIN. [Vol. XIII. 



first is found only in a highly modified form of cleavage ; the 

 two latter are characteristic, so far as is certainly known, of 

 all gasteropods, lamellibranchs, and annelids, and if the thesis 

 mentioned above is to be maintained, these differences must 

 be explained. This, I believe, can be done, and in the follow- 

 ing manner : 



In Crepidula and all other animals with spiral cleavage in 

 which there is a marked difference in size between the macro- 

 meres and micromeres, the first and second cleavage planes 

 represent in the main the furrows between four entoderm cells. 

 These cells may have different axial relations in different cases, 

 hit so far as known the ectoderm and mesoderm cells to which 

 they give rise always have the same axial relations. 



Thus no exception is known either among mollusks or anne- 

 lids to the rule that the second and fourth quartettes lie in the 

 future median and transverse planes, and that the first, third, 

 and fifth quartettes lie midway between these planes. The 

 axial differences, therefore, of the first two cleavage planes, 

 which have been mentioned, are differences ffierely in the axial 

 relations of the four priinary entoderm cells, and do not affect 

 the axial relations of the other cells of the ovum, which are 

 always the same among annelids and mollusks} 



That this is really true is further shown by the fact that in 

 all cases in which the first and second furrows coincide with 

 the transverse and median planes respectively, the whole of the 

 ectoblast and mesoblast rotates around the &gg axis until the 

 second and fourth quartettes lie in the median and transverse 

 planes ; whereas in all forms in which one macromere is anterior, 

 one posterior, one right, and one left, there is no such rotation 

 {cf. cross in Neritina and Crepidula). In all known cases 

 of spiral cleavage, excepting a few sinistral gasteropods, the 

 micromeres are separated from the macromeres in the same 



1 Lillie ('95) has expressed somewhat similar views on this subject. He men- 

 tions three cases, viz., Clepsine, Planorbis, and Neritina, in which the macromeres 

 are anterior, posterior, right and left respectively ; but he discusses only the first 

 of these cases, and concludes that "at present we are unable to explain why, when 

 widely separated forms agree, nearly related species should show reversed rela- 

 tions." The other two cases, viz., Planorbis and Neritina, show clearly that these 

 reversed relations concern only the four macromeres. 



