416 MARGARET ROBINSON. 



from the central mass which forms the anterior part of the 

 deuterocerebrum (fig. 57). 



The Deuterocerebrum now consists of three distinct 

 regions : 



(a) An anterior, median, bilobed mass lying directly be- 

 hind the central mass of the protocerebrum (fig. 57, cm.). 



(b) Two large outer lobes which lie outside of and slightly 

 behind the hinder part of (a), and which are in direct con- 

 tinuity with — 



(c) The ganglia of the antennules. These are slightly 

 larger than they were in Stage E, 



Tritocerebrum. — This has grown considerably and each 

 ganglion now consists of two parts : 



(1) A ventral mass of nerve cells lying close to the 

 stomodaeum, and — 



(2) A larger dorso-lateral mass lying farther from the 

 middle line (fig. 60). 



These masses are in continuity with each other except at 

 the point on either side where fibres pass from the ganglion 

 to the antenna. The bay between these masses of cells is 

 filled with fibres, and, posteriorly, each ventral mass is con- 

 tinued as a thin cord or chain of cells which connects the 

 antennary ganglion with that of the mandible on the same 

 side. 



There is more mesoderm in the brain than was present there 

 in the last stage. This mesoderm consists of chains of large 

 cells which run more or less obliquely from the stomodseum 

 to the dorsal body wall (figs. 59 and 60). These chains are 

 the forerunners of the bands of connective tissue and muscle 

 which connect the stomodgeum with the dorsal body wall in 

 the later stages. 



The brain here closely resembles that of My sis at a like 

 stage of development. 



In the mandibular ganglion the two dorso-lateral clumps 

 of ganglion cells present in the adult are beginning to make 

 their appearance, and, as in the adult, there are ganglion cells 

 in the centre showing the double origin of this ganglion. The 



