758 The American Naturalist. [September, 
lens-shaped mass (Fig. 12, B and C, im) is really entoderm; this 
layer is Rauber's Deckschicht, which, as already stated, usually 
disappears, leaving the true inner mass, or permanent ectoderm, 
to form part of the surface of the blastodermic vesicle, so that, 
with the exception of the reduction in the dimension of the 
entoderm, the relations are the same as in other vertebrate ova. 
The inner layer of the flattened inner mass gives rise to the 
entoderm, and this at first sight appears to be conclusive evidence 
against the homology here drawn between the inner mass and 
the primitive ectoderm of other vertebrates. The same thing 
was formerly supposed to occur in the blastoderm of other verte- 
brates, but it is now known that the entoderm is added from 
another source to the under side of the primitive blastoderm or 
ectoderm, and though we possess no exact information whatever 
as to the origin of the entodermic cells under the primitive blas- 
toderm of the Mammalia there is no reason to assume that 
they arise in a manner fundamentally different from that typical 
of other vertebrates. We may therefore dismiss this objection. 
The origin of the entodermic cavity I hope to discuss on another 
Planes of Division during Segmentation. — The plane of the 
first divisions determines those of the subsequent divisions, and 
also of all the axes of the embryo.^ It is itself determined by 
the position of the long. axis of the first amphiaster or nuclear 
spindle, to which it is at right angles. It, therefore, is a matter of 
great interest to ascertain what factors determine the position of 
the first spindle, or, in other words, the axis of elongation of the 
segmentation nucleus. So far as at present known there are two 
factors: I. Relation to the axis of the ovum; II. Position of 
the path taken by the male pronucleus to approach the female 
pronucleus. The axis of the ovum is fixed before impregnation. 
It passes through the center of the animal and that of the vege- 
table pole. Usually the nuclear spindle which leads to the for- 
