472 PA TTEN AND HAZEN. [Vol. XVI. 



The Branchial Nerves. — In studying sections of young 

 embryos the mesodermic ring is often found fused with the 

 ectodermic wall of the appendage, so that it is impossible to 

 distinguish between them. In some of these places were found 

 what appeared to be large ganglionic or sensory cells, no doubt 

 derived from the overlying thickenings of the ectoderm. They 

 differed in general appearance and coloring from the surrounding 

 cells, and were most numerous at the posterior margin of the 

 appendages, between the mesodermic ring and the ectoderm. 

 In older specimens nerve cells and fibers were found in corre- 

 sponding positions (PI. XXIII, Figs. 25 and 26, «.). 



In a later stage (PI. XXIII, Figs. 30 and 31) the nerves are 

 clearly outlined in both the operculum and the first gill. In a 

 still older embryo (PI. XXIII, Fig. 33), on the posterior side of 

 the first gill, is a proliferation of nerve cells and fibers resem- 

 bling a sense organ. It represents the same proliferation seen 

 in PI. XXIII, Figs. 25-27. From it is formed the first gill leaf. 

 The nerve fibers in question are the large ventral nerves that 

 supply the abdominal appendages. They appear to develop, 

 therefore, as fibrous outgrowths (of the central nervous sys- 

 tem .'') that receive at various points along their course fibers 

 and cells from the overlying ectoderm, and especially from 

 those points where the gill leaves are about to appear. 



VII. The Nephric Gland. 



A. The Development of the Nephridial Lobes. — The nephrid- 

 ial lobes are formed from loose clusters of cells, situated near 

 the base of each of the six thoracic appendages. The cells 

 show a distinctly segmental arrangement, and are derived from 

 what appears to be the median somatic wall of the mesoblastic 

 somites. The nephridial cells of the first and sixth appendages 

 degenerate and disappear after the second larval stage. Those 

 in the second, third, fourth, and fifth appendages form the 

 nephridial lobes of the adult. 



The nephric duct arises in quite a different manner, as a 

 tubular diverticulum of the somatic wall of the fifth thoracic 

 somite ; the nephridial cells of this segment do not appear till 



