KOFOID: DEVELOPMENT OF LIMAX. 69 
of the early stages depends on the estimated amount of shifting of the 
mesoderm cell upon the basal quartet a^! — q^, 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 for Nereis cannot be decided from the evidence 
at hand. It is however suflicient 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 
hoterostropha, whose segmentation likewise shows the same phenomenon 
of reversed cleavage, as I can affirm from my own observations on the 
living egg.! In my former paper (794, p. 191) I called attention to the 
fact that Haddon (782, Plate XXXI. Fig. 6) figures a four- to eight-cell 
stage of Janthina 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. 775) Janthina is a dextral form. 
These cases of Planorbis, Physa, and Janthina at once raise a most inter- 
esting question as to the relation that, may exist between the two forms of 
cleavage, normal and reversed, aud 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 Haddon’s 
is sufficient to throw much doubt on that conclusion. There remains the 
possibility that on renewed examination the cleavage of Janthina may 
prove to be normal, and that this case of Haddon’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 
Janthina; 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 hetorostrophic species, ought to be 
decisive upon this point, where we have as yet so little light. 
1 Consult also the Addendum, page 111. 
