208 General Notes. ` [March, 
sinistral. Of M. rufus, about one and one-half per cent. of every 
one thousand were thus reversed, while the per cent. of M. decisus 
was between two and two and one-half in each hundred. Com- 
paring these averages with the number of mature reversed speci- 
mens collected through quite a long period of time, it was found 
that only about one-tenth of oxe per cent. survived the accidents 
consequent on station and environments. 
How to account for the presence of sinistral shells at all now 
became the problem. I submit the following suggestions: Many 
adult and impregnated specimens were dissected and carefully 
studied, with the result that the position of the embryonic shells 
was such as to necessarily crowd them one on another. As they 
increased in size (this is based upon the inspection of shells in 
different stazes of development), their proximity influenced their 
assumption of form, more and more, and many curious and 
abnormal shapes were given the growing shells. Binney (1. c., p. 
49) figures some of these forms, while others have been described 
as species (e. g. Paludina (Melantho) genicula Con.). Mr. Binney 
very properly groups these aberrant forms under M. decisus or 
M. integer. These “shouldered” and otherwise deformed shells 
are due to the crowding mentioned above. Isit not possible that 
the reversed forms originate in a similar way; the embryonic 
shell increasing in the direction of the least, or no resistance ? 
The direction of the “ whirl” thus started, would be followed in 
all the succeeding stages of development. 
Binney doubts the specific identity of M. rufus Hald., but 
if the usually accepted definition of “ species” be allowed, without 
good reason. The three above-mentioned forms are associated 
in the Erie Canal, at Mohawk, N. Y., and so far as species go 
they are all valid. The latest understanding of a species would, 
however, relegate them all, together with the other southern and 
western forms of the genus, to varieties of one sole type.—X. Zils- 
worth Call, School of Science, Dexter, Towa. 
Laws oF HisrotocicaL DIFFERENTIATION.—In a recently pub- 
lished article (Proc. Boston Soc. Nat. Hist., Vol. xx, p. 202) Dr. 
C. S. Minot discusses certain laws of histological differentiation. 
He maintains that, first, the most primitive form of tissue is an 
epithelium composed of a single row of polyhedral cells of equal 
height. Second, very early in the course of development the ecto- 
dermic cells become smaller and multiply faster than the cells of 
the entoderm. Third, the two horizontal axes of an epithelial 
cell (or those parallel to the surface of the epithelium) usually _ 
remain approximately equal to one another in length, while the — 
perpendicular axis varies independently and to a much greater 
extent. Fourth. epitheliums increase their surface by the forma- 
tion of depressions (invaginations) or of projecting folds (evagina- 
tions). Fifth, structural modifications of epitheliums usually 
affect similarly a whole cluster or tract of cells, but rarely isolated — 
