410 MARGARET ROBINSON. 



System. — The arrangement of the nervous elements which 

 form the brain is very similar to that described in the last 

 stage. The large ganglion cells still form part of the general 

 ectoderm of the body. The protocerebrum still consists of 

 two large lateral and two small median lobes (fig. 44), but 

 the central mass reaches farther forward than it did, only the 

 most anterior region of the protocerebrum being now without 

 this central mass between the optic ganglia. 



The most anterior parts of the optic ganglia resemble those 

 in Stage D, i. e. they consist merely of the two kinds of nerve- 

 cells there described. 



The deuterocerebrum has grown considerably, and now 

 consists of two parts : 



{a) A median mass of nervous cells lying in front of the 

 first antennjB. 



(h) A pair of ganglia lying on a level with, and inner- 

 vating the first pair of antennte. 



The anterior of these parts is, in reality, formed by that 

 part of the central mass of nerve-tissue which lies imme- 

 diately behind the optic region. 



Reichenbach (1886) in describing the brain of a similar 

 stage in the Crayfish says that the anterior parts of the supra- 

 oesophageal ganglia are separated from their posterior parts on 

 the one hand, and from the optic ganglion on the other by 

 strands of connective tissue. Now, in a transverse section 

 through the first antenna in Stage D (fig. o6, f.) there are to 

 be seen cells which partly separate the median mass from the 

 ganglion on either side, but these cells have the appearance 

 o£ ectoderm cells. They do not resemble the mesoderm cells 

 of Stage D, and, as yet, there is no mesoderm in the ventral 

 part of the brain. In Stage E it is only just beginning to 

 enter the brain from the dorsal, i.e. the yolk side. 



Further, the cells lining the slight groove which separates 

 the two lateral halves of the deuterocerebrum ventrally are 

 certainly ectodermal. They can be seen forming a slight 

 inpushing in fig. 47, and also as wedge-shaped cells in figs. 

 50, 4G, and 34 {wed.). 



