214 JOURNAL OF ENTOMOLOGY AND ZOOLOGY 



and first and second antennal ganglia, as well as the more central 

 part of the brain, which is made up of several parts. Into the 

 subesophageal ganglion are fused the lateral parts of the mandibu- 

 lar, two pairs of maxillae and the pair of maxillipeds. Into the 

 thoracic ganglia there seem to be fused two lateral pairs of ganglia 

 for each segment. In the abdominal region the ganglia are not so 

 clearly made out at all stages. Each ganglion is composed of two 

 lateral halves. There are at one time probably a large number of 

 segments much alike as to their ganglia. The abdominal ganglia 

 become massed closer and closer together and fuse into practically 

 one mass. 



McMurrich speaks of an extra appendage without a ganglion in 

 the head region of isopod embryos, about opposite the mandibles. 

 I am not sure but this is the appendage which has been crowded 

 up from below, yet I have found an extra appendage in about this 

 region in certain stages. A number of investigators also speak of 

 an extra ganglion near the brain and separate from the one of the 

 first antenna. I have seen this in a number of stages. It seems 

 like a center of cell growth much like several others. 



In Figs. 28, 29, and 31, three different stages in the development 

 of the brain and optic lobes are shown. Whatever the different cell 

 groups may mean, it is possible to trace the development of the 

 areas from one stage to another. Three main parts of each lateral 

 brain and optic lobe region develop quite early. This is due in 

 part to the different directions of cell growth and also to the differ- 

 ent rates of growth in different areas. Figs. 30 and 32 are dif- 

 ferent stages in the development of ganglia viewed from the ventral 

 side. Figs. 31 and 33 are from the same specimen of a rather 

 advanced stage, such a stage as shown in Fig. 14. 



The gradual development of the shape of the ganglia, their 

 differentiation from the ectoderm of the surface and the develop- 

 ment of the fibrous portions are shown in the drawings of sec- 

 tions of Figs. 33 to 49. The thoracic and abdominal ganglia of 

 a stage such as 11, is shown in cross-section in Figs. 33 and 34. 

 Fig. 35 is a longitudinal section through the ganglion in the middle 

 line of a similar stage. Figs. 48 and 49 are longitudinal and cross- 

 sections through ganglia of stage 13 or 14, ventral side up, areas of 



