JOURNAL, OF ENTOMOLOGY AND ZOOLOGY 213 



Each segment of the body has its ganglion at an early time. 

 This segment between thorax and abdomen which loses its append- 

 age has a well developed ganglionic group or mass. 



As the optic lobes and brain grow in size, they come to take up 

 a more dorsal position, as seen in the figures. After a time pigment 

 begins to be evident in the lower edge of the eye, as shown in Figs. 

 22 and 23. When the embryo or young has its eye fully pig- 

 mented, the little animal is much like the adult. 



The preceding outline of development so far given presents the 

 method of development of the embryo as a whole as well as the 

 gradual increase in size of the ganglia, optic lobes and brain. The 

 development of the cells which take part in this formation of the 

 appendages and nervous system is better shown in whole mounts 

 or sections. 



McMurrich, Nusbaum and others show clearly the development 

 of the post-naupliar region of isopods from teloblastic growth. 

 I hav'e very little to add to their complete accounts. Fig. 24 shows 

 a surface view of an unstained egg in which is a half circle of 

 teloblast cells near the center of the early cell area. Fig. 26 shows 

 a later stage of an embryo. The teloblast cells which form the 

 ectoderm are much enlarged and the six middle rows indicate the 

 cells which develop the nervous system. Fig. 25 is from the nerv- 

 ous system of a similar early stage; the lower appendages and the 

 nervous area between are of teloblastic formation, while the 

 cephalic broader portion of the embryo was probably chieHy formed 

 from sells in situ. In Fig. 27 the teloblasts are still adding material 

 at the caudal end of the embryo. Several ganglionic areas are 

 more or less distinct. Sections of stages similar to this show little 

 differentiation between cells of the general surface and the nervous 

 system. In later stages the ectodermal cells multiply to form the 

 ganglia, and the thin line of surface ectoderm becomes separated 

 and splits off. 



With the growth of the embryo the cells of the brain and ganglia 

 become marked off from the other cells. A ganglionic area from 

 each of the appendages in the head region is evicient, and in addi- 

 tion some cell groups which seem separate, as for instance one near 

 the first antennal ganglion. Into the brain are fused the optic 



