DEVELOPMENT OF THE NERVOUS SYSTEM. 225 



depression arises, and its cells form a mass which con- 

 nects these two cords in front of the mouth, and gives 

 rise to the cerebral ganglia. The epiblastic linings of 

 two small pits (fig. 58, o) which appear very early on the 

 surface of the procephalic lobes, are also carried inwards 

 in the same way, and, uniting with the foregoing, 

 produce the optic ganglia. 



The cells of the longitudinal cords become differ- 

 entiated into nerve fibres and nerve cells, and the latter, 

 gathering towards certain points, give rise to the ganglia 

 which eventually unite in the middle line. By degrees, 

 the ingrowth of epiblastic cells, from which all these struc- 

 tures are developed, becomes completely separated from 

 the rest of the epiblast, and is invested by mesoblastic 

 cells. The central nervous system, therefore, in a crayfish, 

 as in a vertebrated animal, is at first, as a part of the 

 ectoderm, morphologically one with the epidermis; and the 

 deep and protected position which it occupies in the adult 

 is only a consequence of the mode in which the nervous 

 portion of the ectoderm grows inwards and becomes 

 detached from the epidermic portion. 



The visual rods of the eye are merely modified cells of 

 the ectoderm. The auditory sac is formed by an involu- 

 tion of the ectoderm of the basal joint of the antennule. 

 At biuth it is a shallow wide-mouthed depression, and 

 contains no otoliths. 



Lastly, the reproductive organs result from the segre- 

 gation and special modification of cells of the mesoblast 



