of the Annelids and Vertebrates. 465 



dorsal position of the siipraoesoj)hageal ganglion, is contained, 

 according to our mode of tz-eatment, in the first or fourth. 

 Could it be proved that it originates dorsally and indepen- 

 dently of the ventral cord, then its position would be dorsal. 

 Now Gegenbaur asserts in the most decided way that this is 

 proved ; but this is only the case in his own imagination. 



Not a single observation on the Articulates has been made 

 which really satisfactorily demonstrates that it is formed on 

 the dorsal side ; while some, on the other hand, prove very 

 exactly that it takes its origin from the ventral side. Blitschli 

 has shown in the bee, and anin still more clearly in the larva? 

 of Ichneumonida3, that the anterior end of the first rudiment 

 of the ventral cord divides into two parts, which grow up- 

 wards round the oesophagus, and only unite dorsally at a later 

 stage to form the so-called brain. No one mentions the ap- 

 pearance of a separated medullary plate of the dorsum in the 

 Articulates ; the frontal plates [Scheiteljylatten) lie at first on 

 the ventrum, and only gradually reach the dorsum. The as- 

 sertion of various observers that these arise on the dorsum 

 proves, from their own statements, that they have not under- 

 stood the first developmental stages. 



I can confirm the observations (only made, however, inci- 

 dentally) of Biitschli and Ganin in the most decided way as 

 regards the Naidere, in which I have studied the formation of 

 zooids uninterruptedly for six months, with the intention of 

 clearing up the primary origin of the nervous system (ventral 

 cord and brain). I have already gone far enough in this in- 

 vestigation to be able to bring forward the following points as 

 firmly established. 



1. The ventral cord originates neither exclusively in the 

 ectoderm (Kowalevsky) nor in the mesoderm (Leuckart, 

 Eathke), but both layers take part in its formation. Only 

 the central azygos ganglion [Clepsine) or the azygos cellular 

 cord under the nervous cord [Lumhricus &c.) originates di- 

 rectly in the ectoderm ; and this is primitively quite unseg- 

 mented, precisely as in the osseous fishes. The two lateral 

 ganglia, however, arise out of the protosegments of the meso- 

 derm, and are therefore segmented from before backwards. 

 The first-mentioned central ganglion alone corresponds to the 

 spinal cord of Vertebrates, while the lateral ones correspond 

 to their spinal ganglia. 



In agreement with this, the lateral nerves leaving the gan- 

 glionic chain arise by two roots ; they are true spinal nerves. 

 Herrmann has clearly distinguished these two roots in the 

 leech as superior and inferior. 



2. The muscle-plate appears at first not in the neural (ven- 

 Ann. d- Mag. X. Hist. 8er. 4. Iv)?. xvii. 31 



