308 GROWTH IN LENGTH OF THE EMBRYO. 



to take place by concrescence. If therefore His' and Rauber's view is accept- 

 ed, it will have to be maintained that only a small part of the body is form- 

 ed 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 blas- 

 toderm ; 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 (i) 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 surprising 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 ap- 

 pears 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 arguments 

 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. 4). 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 

 Vertebrates the ventral walls of the body are obviously completed by the 

 closure of the lips of the blastopore, on the ventral side. 



If His and Rauber are right the dorsal walls are also completed by the 

 closure of the blastopore, so that the whole of the dorsal, as well as of the 

 ventral wall of the embryo, must be formed by the concrescence of the lips of 

 the blastopore ; which is clearly a reductio adabsurdum of the whole theory. 

 To my own arguments on the subject I may add those of Kupffer, who has 

 very justly criticised His' statements, and has shewn that growth of the 

 blastoderm in Clupea and Gasterosteus is absolutely inconsistent with the 

 concrescence theory. 



The more the theory of His and Rauber is examined by the light of com- 

 parative embryology, the more does it appear quite untenable ; and it may 

 be laid down as a safe conclusion from a comparative study of vertebrate 



