180 The American Naturalist. [February, 
EMBRYOLOGY. 
Development of an Isopod—The first paper of M. Louis Roule 
on the development of the Crustacea has just appeared.” He has 
studied, as a representative of the Edriopthalmia, the Isopod, Por- 
cellio scaber, Leach, with especial attention to the first stages of devel- 
opment. The origin of the blastoderm, of the germ layers, and of the 
rudiments of the organs, are considered in great detail, and there is, 
besides, much general discussion on the significance of these processes 
among Arthropods. This first paper will be followed by three others ; 
one on the Decapods, one on the Copepods and Branchiopods, and a 
third on general questions. 
Though Bobretzky, in 1874, established the chief features of the 
development of Isopods from a study of Oniscus murarius, a detailed 
examination of the development of the group was much needed ; and 
M. Roule has also wished to throw more light on the question of rela- 
tionship between Annelids and Arthropods. He has been led to be- 
lieve that the early stages of the Crustacea do aid us materially in 
testing such an affinity. 
The eggs of Porcellio scaber develop in the brood chamber of the 
mother, the early stages (including segmentation, the formation of 
the blastoderm, and establishment of the germ layers) requiring a 
proportionately long time, about two weeks, while the rest of embry- 
onic development takes but three weeks. 
The unsegmented ovum is mainly a mass of nutritive yolk, with the 
greater part of the formative material on the surface in the form of 
“islands” of protoplasm. The food yolk consists of a great number of 
large vitelline granules in a protoplasmic groundwork. Toward the 
periphery the granules are smaller, and the islands of formative yolk, 
though finely granular, are mostly formed of the protoplasmic ground- 
work. In a surface view one of the islands of protoplasm is found 
constantly at the anterior pole, and is seen to be larger than the others. 
This is the germinal disc, and it contains the only nucleus in the 
ovum. The other islands have no constant position or size; they are 
continuous with the central deutoplasm (as is also the germinal disc), 
1 Edited by E. A. parma Baltimore, Md., to whom abstracts, reviews and 
preliminary notes may be sen 
aes des Sciences ai, Vol. xviii, Nos. 1,2,3 (sér. 7), with 10 
plates 
