470 The American Naturalist. [June, 
surrounding a segmentation-cavity, but the resemblance to 
the typical blastula is masked by changes in both ectoderm 
and entoderm ; the vertebrate ectoderm when first fully differ- 
entiated consists of several layers of cells, and not merely of 
a single layer of cells as in the primitive type of segmenta- 
tion ; the entoderm contains a very large amount of nutritive 
material (deutoplasm) and is represented either by a mass of 
large cells (marsipobranchs, ganoids, amphibians) or a mass 
of protoplasm, not divided into cells, or but partially divided 
into cells, and containing an enormous quantity of deutoplasm 
(sauropsidans and monotremes). In the higher mammals 
there are further modifications as described below. 
The more primitive form among vertebrates is, I think, pre- 
sumably, that in which the entoderm consists of separate cells, 
for this mode of segmentation is the one which most resembles 
that of invertebrates, and it occurs in the lowest vertebrates, 
and in ova which are not excessively charged with yolk. 
In the primitive form of vertebrate segmentation, which is 
preserved in the marsipobranchs, ganoids and amphibians, 
there is a well marked difference between the cells of the two 
poles. The following account refers especially to the frog's 
Qgg, and is an adaption of Balfour's summary (Comp. Embryl. 
I-. 78, 79). The first formed furrow is vertical ; it commences 
in the upper half of the ovum which corresponds to the animal 
pole and is characterized by the black pigment — the lower or 
vegetable pole being whitish. The first furrow extends rapidly 
through the upper, then more slowly through the lower half 
of the ovum, so that the divergence in the two polar rates of 
development is indicated already. As soon as the furrow has 
cleft the Qgg into halves, a second vertical furrow appears at 
right angles to the first and behaves in the same way (Fig. 3)* 
The next furrow is' at right angles to both its predecessors, 
and therefore parallel to the equator of the egg, but it is much 
nearer the animal than the vegetative pole. It extends rapidly 
around the egg and divides each of the four previous segments 
into two parts ; one larger with a great deal of yolk, and the 
other smaller with very little yolk. The eight segments or cells 
