kofoid: development of li.max. 69 



of the early stages depends on the estimated amount of shifting of the 

 mesoderm cell upon the basal quartet a 71 — d 71 . There is evidence in 

 Rabl's figures ('80, Taf. XXIX. Figg. 2, 3, 4) that there is at least some 

 shifting, but whether it is sufficient to justify an orientation similar to 

 that adopted by Wilson fur Nereis cannot be decided from the evidence 

 at hand. It is however sufficient to raise the question as to the cor- 

 rectness of Rabl's orientation. 



In Planorbis the asymmetry of the adult is sinistral ; so also in Physa 

 heterostropha, whose segmentation likewise shows the same phenomenon 

 of reversed cleavage, as I can affirm from my own observations on the 

 living egg. 1 In my former paper ('94, p. 191) I called attention to the 

 fact that Haddon ('82, Plate XXXI. Fig. 6) figures a four- to eight-cell 

 stage of Jauthina which apparently presents a left spiral, i. e. the reverse 

 of that found in a corresponding stage of Limax and other dextral forms. 

 But according to Fischer ('80-87, p. 77-3) Jauthina is a dextral form. 

 These cases of Planorbis, Physa, and Jauthina at once raise a most inter- 

 esting question as to the relation that may exist between the two forms of 

 cleavage, normal and reversed, and the dextral or sinistral asymmetry of 

 the adult. The occurrence of reversed cleavage in these two sinistral 

 forms is suggestive of a causal relation between the conditions of the em- 

 bryo and the adult, but the existence of this one observation of Haddou's 

 is sufficient to throw much doubt on that conclusion. There remains the 

 possibility that on renewed examination the cleavage of Jauthina may 

 prove to be normal, and that this case of Haddou's may be referred to a 

 chance sinistral form. In answer to an inquiry of mine, Dr. J. I. Peck 

 of Williams College has kindly written, " I have never seen a sinistral 

 Jauthina; all of those which came under my observation ('93) were 

 dextral : all of those in our collection here are dextral also, as were 

 those I saw at the Museum at Yale. These were of four species, I think. 

 and collected at different points of the Atlantic and Pacific, both Arctic 

 and Tropical. I have, however, probably not seen enough material to 

 expect a sinistral individual." Of the two or three hundred shells of 

 Janthina, representing several species, in the Museum collection, not one 

 is sinistral. 



The data for the discussion of this interesting question are as yet too 

 scanty to justify any generalizations. The examination of the cleavage 

 in such genera as Fulgur and Achatinella, which contain both dextral 

 and sinistral forms, as well as that of heterostrophic species, ought to be 

 decisive upon this point, where we have as yet so little light. 



1 Consult also the Addendum, page 111. 



