322 WILLIAM PATTEN. 



way in front of the mouth, forming a sharply defined region, 

 that I shall call the mid-brain (Fig. 1, m. b.). 



The remaining five thoracic neuromeres are imperfectly 

 fused; they constitute the hind-brain (h. b.). Finally, four 

 very intimately fused abdominal neuromeres are added to the 

 preceding ones, forming an accessory brain (a. b.). 



A very similar grouping is found in Vertebrates. (1) As 

 shown by the segmental character of the optic, pineal, and 

 olfactory nerves, the fore-brain probably contains at least three 

 completely fused neuromeres. (2) The mid-brain, as is now 

 generally recognised, contains but a single neuromere, which, 

 judging from the character of its nerves and somite, probably 

 belonged originally to what Gegenbaur calls the six primitive 

 head-segments, and which, just as in Arachnids, has subse- 

 quently become separated from them, forming an independent 

 region. (3) The hind-brain is composed of five or six neuro- 

 meres, which Gegenbaur, omitting the fore and accessory brain, 

 regards as the primitive brain ; the large size of these neuro- 

 meres in Vertebrates, their incomplete fusion, and the distinct 

 swellings at an early stage in this brain region, are facts to 

 be expected on the Arachnid theory, for these features are also 

 characteristic of the six thoracic neuromeres of Scorpio and 

 Limulus. (4) According to Balfour and Van Wyhe, there is 

 an accessory brain in Vertebrates composed of four body 

 neuromeres, secondarily added to the head. 



There is difference of opinion as to the exact number of 

 neuromeres in each brain region of Vertebrates ; but as the 

 matter now stands it would not violate these views more than 

 they do one another to assume that the grouping of cranial 

 neuromeres in Vertebrates is exactly the same as in Scor- 

 pions. 



II. Spinal Nerves. — In embryo Scorpions each neuromere, 

 except those of the fore-brain, has three pairs of nerves ; one 

 pair is mainly motor, another mainly sensory, and the third is 

 probably sympathetic. 



In the abdominal region the nerves to each neuromere fuse 

 to form the spinal nerves of the adult; but the distal and 



