168 DR DOIIIIN S HYrOTHESIS. 



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 struc- 

 tures 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 before 

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

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

 supposition of the oesophageal 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 struc- 

 ture 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 ven- 

 tricle to be the part of the nervous system which previously 

 formed the oesophageal 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 indistinguish- 

 able from other parts of the nervous system, and only thins 

 out at a later period. Further than this, any explanation 

 of the thin roof of the fourth ventricle ouojht also to elucidate 



