NERVOUS SYSTEM. 



163 



of these ganglia, which develops in the proximal portion of the 

 eye-stalk, yields the optic ganglion (o', o"), while the remaining two 

 belong to the first and second pairs of antennae and enter into 

 the formation of the brain proper or supraoesophageal ganglion. Of 

 these last, that lying in the segment of the first antenna (antennule) 

 very soon becomes divided by a transverse constriction into a pair of 

 .ganglia (a, b), the anterior of which (a) we will distinguish by 

 PACKARD'S name of jwocerebrum, while the posterior, since it gives 

 off the nerve to the first antenna, has been called the antennular 



Fio. S3. Embryo of Astacus Jluviatllls with the rudiments of all the limbs (after REICHENBACH). 

 a, anterior, b, middle, c, posterior portion of the rudiment of the brain ; aft, abdomen ; an', 

 first antenna; an", second antenna; d, mandibular ganglion; md, mandible; mx' t first 

 maxilla ; mx' 1 , second maxilla ; mf, mf", mf", first, second, and third maxillipedes ; o, rudi- 

 meut of the compound eye; o', the part of the optic ganglion which has arisen from the 

 optic imagination ; o", inner part of the optic ganglion ; ol, upper lip; r, rudiment of the 

 rostrum ; t, telson ; th, fold of the thoracic shield ; I-V, first five pairs of ambulatory limbs. 



fjancjlion (I). It should be mentioned that REICBENBACH believed 

 that he observed, in late stages, a similar transverse division of the 

 posterior pair of ganglia which give off the nerves to the second 

 antennae, and are therefore called the antennal garujlia (c) ; this 

 latter division, however, is not so striking, and also not of the same 

 significance as the division which takes place in early stages between 

 the procerebrum and the antennular ganglion. 



It is an important fact that the pairs of ganglia just described 



