186 The American Naturalist. [February, 
consists in splitting a central layer of cells belonging to the food yolk 
_ from a superficial ectoderm. The protentoderm does not arise, accord- 
ing to this, from the ectoderm, but both layers are the result of a divi- 
sion of the blastoderm. Just as in any planula of this sort (lecithal), 
there is an external epithelial layer of cells, while the internal ones are 
connected with the yolk 
Other Crustacea show a simpler and more typical planula. Here, 
after a total segmentation resulting in a collection of pyramidal cells, 
the central ends containing the yolk divide off, forming an inner layer 
of cells (protendoderm), while the peripheral remains as the ectoderm. 
In the Isopods the mesentoderm separates from the yolk mass as 
ameeboid cells. In insects the condition is more complicated, but es- 
sentially the same. The cells produced from the formative yolk travel 
to the periphery and become the blastoderm, which divides as in the 
Crustacea into the mesentoderm and ectoderm. The cells from the 
centre, which do not reach the blastoderm before its splitting into two, 
are really blastoderm cells retarded from becoming ectoderm. They 
become mesentoderm cells. M. Roule distinguishes them by the term 
“inner blastoderm.” 
“ By whatever method formed, the resulting planula is composed of 
a peripheral blastoderm and a central deutoplasm. Itis centrolecithal. 
This planula is peculiar to Arthropods and some Hirudinea.” 
“ The lecithal planula of other animals belong to Cephalopods, some 
Tunicates, and many vertebrates (Teleosts, Selachians, Sauropsida). 
Here, too, the central yolk mass is enclosed by a disc gradually spread- 
ing over the surface; but in these cases the blastoderm divides into 
two parts, one thick, situated at the place where the disc started to 
spread, and alone destined to give rise to the embryo, while the rest is 
reduced to a thin membrane, limiting the yolk and absorbing it, but 
not forming any part of the adult. These two portions of the blasto- 
derm are the embryonic zone and the vitelline zone. They are con- 
tiguous, and the nutritive mass is not placed in the interior of the young 
individual. In Arthropods the condition is quite different. The blas- 
toderm is entirely embryonic, and encloses all the deutoplasm which 
: forms an internal vitelline vesicle, and not a contiguous one. The 
centrolecithal condition of the planula and the genetic unity of the 
blastoderm so constant among Arthropods, lends to these creatures an 
autonomy separating pen RE other oœlomates: n 
The germ layers of Art d ding to M: Roule, homo- 
logous with those of other gokil. The ectoderm, however, he be- 
lieves to be homologous in origin and history in all cases. Homology 
