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ganglion-cord appears much contracted. In the fourth stage 

 it was continued far into the abdomen, now we onlv meet 

 with it in the cephalothorax. 



A great advance on the fourth stage is also to be 

 observed in the development of the ganglia. In Stage IV 

 it was impossible to distinguish them from each other; 

 now they are very distinct. The whole brain has become 

 very similar to that of the adult. 



The outer form is to be seen in the illustration (fig. 2), 

 made after a model constructed by me from a section - 

 series. I will begin by a description of the superficial 

 appearance of the brain. 



A glance at the illustration will show that the brain 

 consists of two parts, a smaller dorsal and a larger ventral. 

 It is pierced by the alimentary canal in the region where 

 the two parts are joined to each other. 



Five pairs of nerves issue from the ventral or post- 

 oral part of the brain ; these belong to the pedipalpi and 

 the legs. The nerves and ganglia belonging to the cheli- 

 cerse are prseoral. 



No other nerves at yet leave the dorsal portion of the 

 brain, though two swellings on each side of it, just behind 

 the ganglion of the chelicerse, denote the optic ganglia. 

 They are not yet connected with the eyes by nerves, these 

 being very late to appear. The dorsal portion of the central 

 nervous system contains the accessory brain, the four 

 cerebral ganglia, and part of the ventral ganglion-cord, 

 containing the ganglia of the chelicerse. 



The ventral ganglion-cord consists of eighteen ganglia, 

 six belonging to the extremities. The other twelve are 

 very small; although lying in the cephalothorax they really 

 belong to the abdomen. 



The elements forming the brain and ventral ganglion- 

 cord are histologically mostly the same as those described 

 in the fourth stage. We now remark, however, small 

 masses of darker stained fibres in the commissural parts 

 of the central nervous system, the origin and structure of 

 which it is difficult to understand. They are not to be 

 found in the adult brain. 



