39O DEVELOPMENT OF ELASMOBRANCH FISHES. 



canal was any longer enclosed by a commissure of the central 

 nervous system. With the change of mouth Dr Dohrn also sup- 

 poses that there took place a change, which would for a swim- 

 ming animal be one of no great difficulty, of the ventral for the 

 dorsal surface. This general explanation of Dr Dohrn's, apart 

 from the considerable difficulty of the fresh mouth, appears to 

 me to be fairly satisfactory. Dr Dohrn has not however in my 

 opinion satisfactorily dealt with the questions of detail which 

 arise in connection with this comparison. One of the most 

 important points for his theory is to settle the position where 

 the nervous system was formerly pierced by the oesophagus. 

 This position he fixes in the fourth ventricle, and supports his 

 hypothesis by the thinness of the roof of the spinal canal in this 

 place, and the absence (?) of nervous structures in it. 



It appears to me that this thinness cannot be used as an 

 argument. In the first place, if the hypothesis I have suggested 

 as to the formation of the spinal canal be accepted, the forma- 

 tion of the canal must be supposed to have occurred in point 

 of time either after or before the loss of the primitive mouth. 

 If, on the one hand, the spinal canal made its appearance be- 

 fore the atrophy of the primitive mouth, the folding to form it 

 must necessarily have ceased behind the mouth ; and, on the 

 supposition of the cesophageal ring having been situated in the 

 region of the fourth ventricle, a continuation of the spinal canal 

 could not be present in front of this part. If, on the other 

 hand, the cerebro-spinal canal appeared after the disappearance 

 of the primitive mouth, its roof must necessarily also be a 

 formation subsequent to the atrophy of the mouth, and varieties 

 of structure in it can have no bearing upon the previous position 

 of the mouth. 



But apart from speculations upon the origin of the spinal 

 cord, there are strong arguments against Dr Dohrn's view about 

 the fourth ventricle. In the first place, were the fourth ventricle 

 to be the part of the nervous system which previously formed 

 the cesophageal commissures, we should expect to find the 

 opening in the nervous system at this point to be visible at an 

 early period of development, and at a later period to cease to be 

 so. The reverse is however the case. In early embryonic life 

 the roof of the fourth ventricle is indistinguishable from other 



