and salt water sow-bugs, etc. (Isopods), and the beach fleas (Am- 

 phipods). The development of the Asellus aquations of Europe 

 has been studied by E. Van Beneden. He found that the seg- 

 mentation of the yolk is partial ; that after a blastodermic moult 

 the two pairs of antenna; are formed before the mandibles and 



time the embryo moults again. Like all Tetradecapods the young 



tacea. with the improved means of examination instituted mainly 

 by Kowalevsky, is that of Oniscus mumrius, a sow-bug, by Dr. 

 N. Bobretzky, a student of the eminent Russian zoologist. The 

 following is an abstract of his paper. The egg is provided with a 

 chorion and yolk skin. The first change after fertilization is the 

 origin of the formative or original blastodermic cells, which arise 

 at one pole of the egg. As a result of the self-division of the 

 •"ingle primitive blastodermic cell, there arises a disk corresponding 

 to the primitive streak of other articulates, consisting of a single 

 layer of large spheres of segmentation. It thus appears that the 



Before one-half of the surface of the egg is covered, the middle 

 and inner germ-layers are indicated by a mass of eells in the con- 

 cavity of the outer layer, resulting from the division of some cells 

 Of the outer layer. This primitive mass is the lii-t indication of 



they press, (fie finds this" to be the case also in Crangon and 

 Palaeinon.) There are, then, three germ-layers as in the verte- 

 brates. 



The primitive disk, or streak, then forms by the cells of the 

 outer layer assuming a cylindrical form. The first indication of 

 the intestine is an invagination of the hinder end of the primitive 

 band. A larval skin, like that of Asellus and other Crustacea, 



finds that, contrary to Kowalevsky 4 opinion, the inner germ-layer 



limbs grow out, a cross-section shows that it is due to a bulging 

 out of the outer germ-layer, the cavity being tilled with cells of 

 the middle layer. Now appear the first indications of the liver, a 

 layer of large cells forming the liver sac. After the appendages 



