901 



published, until he had gone beyond the stage in which he could 

 „sich leicht überzeugen, daß sie (die Spinalganglien) aus den peripheren 

 Teilen der Medullarplatte hervorgehen'^ It is by no means so easy 

 to arrive at the true facts in Rana. 



The origin of the nervous system from a nerve sheath which has 

 been advocated by Hubeecht for Nemertines, by Marshall for Echi- 

 noderms, and by Spencer for Vertebrates seems to me to be entirely 

 unsupported by any facts in Vertebrate ontogeny; hence I must 

 insist that the Vertebrate nervous system is not referable to a nerve 

 sheath, but that a mass of evidence appears to me to be accumulating 

 in favour of its origin from such a nervous system as that of the 

 Annelida. But I am in this paper only dealing with the facts of de- 

 velopment, and have no concern with hypotheses, and I will conclude 

 by the description of certain appearances in the formation of the 

 central nervous system which appear to be of the highest importance, 

 more especially in regard to the theory of the Annelidan origin of the 

 Vertebrata. 



II. The development of the medullary tube. 



In most of the recent text books of comparative anatomy and 

 embryology three modes of development of the central nervous system 

 are recognized ^). As types one may take 1. Amphioxus, 2. Teleostei 

 and 3. Elasmobranchs. Many attempts have been made to reduce the 

 Teleostean type to that of Elasmobranchs. Some of them, notably that 

 of Calberla, had their basis in a supposed division of the medullary 

 plate of Vertebrates into two strata, an inner nervous one, and an 

 outer epithelial, the latter having the characters of the rest of the 

 „indiflerent" or ordinary epiblast. The inner layer is said later to 

 develop cilia and to form the central canal. 



Unfortunately there are several mistakes in these views. But re- 

 turning to Calberla; it has been shewn by Ziegler and others that 

 in Teleosteans there is no passage of the Deckschicht into the solid 

 keel, in fact there, as elsewhere, the whole plate is sunk below the epi- 

 blast, and the Deckschicht is not represented in forms other than 

 Cyclostomata, Teleostei, Anura, and perhaps Ganoids. The researches 

 I have here recorded on the peripheral nervous system throw some 

 light on the development of the central. 



They enable me to demonstrate that there is no real difference 



1) WiEDEBSHEiM alone appears not to have attached much import- 

 ance to the three ways. 



63 



