388 0. 0. WHITMAN. 



■wards, the fore part closing as the hinder part opens 

 further back, until ultimately the canal is found in 

 the extreme hind end of the tail. 



That part of the primitive streak that lies originally behind 

 the canal does not atrophy, but forms the tail and allantois; 

 while that part that lies before this point is employed in form- 

 ing the medullary tube, the chorda, and the alimentary canal. 

 If Strahl's statements in respect to the change of position of 

 the neurenteric canal are correct, concrescence beyond its point 

 of origin is not an impossibility. It is quite possible that this 

 movement of the canal may explain, what would otherwise be 

 difficult to understand, the occurrence of several apparently 

 independent canals reported first by Braun. 



The chief difficulty in the case of Amphioxus has already 

 been considered by Rauber in two papers that appeared in 

 1877 (Nos. 39, 41), and I do not readily understand why 

 Balfour should bring up the same point several years later 

 without alluding to its previous notice. In considering this 

 objection it must be borne in mind that Amphioxus is no 

 longer entitled to the rank of '' Urwirbelthier," but is rather 

 to be classed among the prodigal sons of the vertebrates, to 

 use a metaphor of Prof. Dohrn. Why, then, should "clear 

 evidence " be expected from this source rather than from more 

 respectable types that have not lost their senses by hiding in the 

 sand ? If degenerative simplicity of structure warrants such 

 expectation it would hardly appear ridiculous to appeal to our 

 more " degenerate cousins," as Lankester calls the Ascidians. 

 The mere fact that the blastopore nearly closes before the ap- 

 pearance of the medullary groove cannot be accepted as proof 

 that the blastopore is not coextensive with the groove, as 

 Rauber has plainly shown. Rauber's remarks on this subject 

 are substantially as follows : — In order to understand the case 

 of Amphioxus we must not start with the last stage of the 

 blastopore, when it has been reduced almost to minimum di- 

 mensions, but with the highest stage of its existence, when it 

 has its widest expanse. It is about this time that it appears to 

 lie on the dorsal side of the earlier equatorial line of the egg. 



