340 Thos. H. Montgomery. 



more striking difference is that the cephalothorax develops no ento- 

 hlast and of the alimentary tract possesses simply the ectoblastic 

 stomodaeum — it is composed only of ectoblast and mesoblast; a 

 smaller difference is that the cephalothoracal extremities (with the 

 exception of the rostrum) arise at the lateral margins of their seg- 

 ments while the abdominal extremities are more mesial in position. 

 The abdomen persists until a late stage as the more embryonic por- 

 tion. 



These growth differences have been indicated by previous observ- 

 ers, though not specially insisted upon. 



16. Central Nervous System. 

 The nerve ganglia arise later than the somites and are hardly per- 

 ceptible until the first abdominal segments are formed ; they are 

 paired ectoblastic thickenings. There is a separate pair for the 

 chelicera (distinct from the cerebral ganglia), for the pedipalps, for 

 each leg pair, and for each of the seven anterior abdominal segments, 

 or thirteen in all, exclusive of those of the head lobe. The anlages 

 of the central ganglia are the paired cerebral ridges with the groove 

 (fovea) behind each; these are situated at first at the antero-mesial 

 border of each half of the head lobe. On each half of this lobe, but 

 more lateral, develops a little later an invagination with heightened 

 wall, the antero-lateral vesicle, and behind the latter a second smaller 

 pit, the postero-lateral vesicle; ultimately each half of the head lobe 

 comes to possess an obliquely transverse ridge, which forms one com- 

 mon border to the fovea and the lateral vesicles. The fovea and the 

 antero-lateral and postero-lateral vesicles are thus at first separate 

 invaginations, and for that reason might be considered separate 

 neuromeres ; but all three are inpushings of the one common ectoblas- 

 tic thickening, the head lobe, all become subsequently continuous and 

 there is only one mesoblastic somite (the rostral) beneath them; 

 therefore, it seems best to consider the head lobe composed of but one 

 pair of neuromeres. These sink beneath the surface, the cerebral 

 ridges and foveae to form the (most dorsal and posterior) cerebral 

 ganglia, and the antero-lateral and postero-lateral vesicles to form 

 the (more lateral) optic ganglia. At the stage of reversion the 



