of the Annelids and Vertebrates. 467 



matlon of the dorsal oesophageal ganglion, viz. : — the central 

 nervous system derived from the ectoderm of tlie ventral side, 

 and dividing to form the oesophageal ring ; the two lateral 

 spinal ganglia growing round the gullet, and so constituting 

 the greatest part of the oesophageal ring ; and, thirdly, the 

 two sense-plates growing upwards from both sides towards the 

 oesophageal ring. There is no trace, however, of an azygos 

 thickening of the ectoderm, situated in the median line of the 

 dorsum, in which the so-called brain could originate ; this is 

 formed, as is seen, in the most marked contradiction to the 

 authoritative assertion of Gegenbaur, by the coalescence of 

 two primitively completely separated elements, derived from 

 the ventrum. The distinction between the brain and ventral 

 cord of the Articulates is therefore removed. 



With tlie disappearance of this distinction and a reference 

 to the facts, long known but completely ignored by Gegen- 

 baur, that the sense-organs are not connected exclusively with 

 the so-called brain of the Articulates, the arguments of the 

 Heidelberg zoologist fall at once to the ground. The other 

 suggestions that he brings forward against my view are due 

 not to himself, but to Baer. 



In Baer's opposition two arguments of different natures are 

 combined. The one, the " evolutio bigemina," which is only 

 typical for the Vertebrates, is purely morphological ; the other, 

 the distinction of ventrum and dorsum, is purely physiological, 

 or almost completely so, dependent, namely, upon the relations 

 of the united organism to the ground bearing it, or to the nu- 

 triment it seeks. 



The purely morphological argument is refuted by the facts 

 stated above; "evolutio bigemina" is also typical for the 

 Annelids. In these also there are two parts of the animal 

 muscular layer one above the other, and separated by an axis 

 as in the Vertebrates ; and as in these latter, so in the former, 

 the one sui-rounds the alimentary canal, and the other the 

 central nervous system. In the Arthropods tliis type ai^pears 

 to be obliterated. I say expressly appears\ for up to this time 

 the mode of growth of the muscle-plates has never been de- 

 termined by transverse sections; and so it is quite possible that 

 their development takes place in the same way as in the 

 Annelids. 



Further, should any one succeed in demonstrating that, in 

 this group also, two primary blastodermic layers take part in 

 the formation of the ganglionic chain, which is quite possible, 

 the proof of " evolutio bigemina " in the Arthropocls would 

 then be furnished, and the desired correspondence with the 

 Vertebrates established. 



