106 ADAM SEDGWICK. 



of the primitive ring intervening between the mouth and anus, 

 unites more or less completely across the middle line. 



It and the superficial epiblast with which it is in connection 

 become grooved ; the groove becomes deepened and converted 

 into a canal open close to the mouth in front and close to the 

 anus behind (fig. 15). 



The function ^ of this canal at this stage (the siphon stage) 

 I have elsewhere discussed and ventured to suggest that it was 

 in the main respiratory. (For the erabryological counterpart 

 of the siphon stage, see below, p. 75.) 



It is important to notice that the nervous system of the 

 Vertebrata becomes removed from the surface in quite a dif- 

 ferent way to that which obtains in the Invertebrata. In the 

 latter it becomes removed from the surface by the ingrowth of 

 mesoblastic tissue between it and the superficial layer in con- 

 nection with which it arose. In the former, on the other hand, 

 it never separates from the superficial epiblast from 

 which it arises. The latter is involuted with the nervous 

 mass and persists through life as the lining of the canal of the 

 Vertebrate nervous system. This fact is of great importance in 

 speculating on the origin of the Vertebrata, for it shows that the 

 Vertebrate stock is a very primitive one, and must have sepa- 

 rated from the Invertebrate stock before the nervous system of 

 the latter separated from the epidermis.^ 



It will be observed that in consequence of the development 

 of the praeoral lobe (fig. 15 not marked enough), the mouth has 

 become placed on the other side of the body, i. e. on the 

 abneural side, and the neural canal has to bend towards this 

 surface (the future ventral surface) in order to open into the 

 mouth. 



The water which was attracted by the ciliary movement 



' For a discussion of the function of the canal at this stage, vide Sedg- 

 wick, " On the Original Function of the Canal of the Central Nervous 

 System of Vertebrata," ' Proceedings of tlie Cambridge Philosophical Soc.,' 

 vol. iv. 



" This fact also holds for the cerebral ganglia of Peripatus; the invagina- 

 tions of ectoderm become constricted off, and their cavities persist throughout 

 life in the ventral protuberances of the brain. 



