COMPARISON^ OF THE a E KM IN A L LAYERS. 255 



which he adopts is more conclusively proved than by that of any other 

 group. 



(1) The general appearance of the thickened edge of the blastoderm 

 becoming continuous with the medullary folds has been used as an argument 

 for the medullary folds being merely the coalesced thickened edges of the 

 blastoderm. Since, however, the medullary folds are merely parts of the 

 medullary plate, and since the medullary plate ia continuous with the 

 adjoining epiblast of the embryonic rim, the latter structure must be con- 

 tinuous with the medullary folds however they are formed, and the mere 

 fact of their being so continuous cannot be used as an argument either 

 way. Moreover, were the conci^escence theory true, the coalescing edges 

 of the blastoderm might be expected to form an acute angle with each 

 other, which they are far from doing. 



(2) The medullary groove becomes closed behind earlier than in front, 

 and the closure commences while the embryo is still quite short, and 

 before the hind end has begun to project over the yolk. After the medullary 

 canal becomes closed, and is continued behind into the alimentary canal 

 by the neurenteric passage, it is clearly impossible for any further increase 

 in length to take place by concrescence. If therefore His' and Rauber's 

 view is accepted, it will have to be maintained that only a small part of 

 the body is formed by concrescence, while the larger posterior part grows 

 by intussusception. The difficulty involved in this supposition is much 

 increased by the fact that long after the growth by concrescence must have 

 ceased the yolk blastopore still remains open, and the embryo is still 

 attached to the edge of the blastoderm ; so that it cannot be maintained 

 that the growth by concrescence has come to an end because the thickened 

 edges of the blastoderm have completely coalesced. 



The above are arguments derived simply from a consideration of the 

 growth of the embryo ; and they prove (1) that the points adduced by His 

 and Rauber are not at all conclusive; (2) that the growth in length of 

 the greater part of the body takes place by the addition of fresh somites 

 behind, as in Chaetopods, and it would therefore be extremely surpi'ising 

 that a small middle part of the body should grow in quite a different way. 



Many minor arguments used by His might be replied to, but it is 

 hardly necessary to do so, and some of them depend upon erroneous views 

 as to the course of development, such as an argument about the notochord, 

 which depends for its validity upon the assumption that the notochord ridge 

 appears at the same time as the medullary plate, while, as a matter of fact, 

 the ridge does not appear till considerably later. In addition to the argu- 

 ments of the class hitherto used, there may be brought against the His- 

 Rauber view a series of arguments from comparative embryology. 



(1) Were the vertebrate blastopore to be co-extensive with the dorsal 

 surface, as His and Rauber maintain, clear evidence of this ought to be ap- 

 parent in Amphioxus. In Amphioxus, however, the blastopore is at first 

 placed exactly at the hind end of the body, though later it passes up just 

 on to the dorsal side {vide p. 3). It nearly closes before the appearance of 

 the medullary groove or mesoblastic somites ; and the medullary folds have 

 nothing to do with its lips, except in so far as they are continuous with 

 them behind, just as in Elasmobranchii. 



(2) The food-yolk in the Vertebrata is placed on the ventral side of the 

 body, and becomes enveloped by the blastoderm ; so that in all large-yolked 



