254 GROWTH IN LENGTH OF THE EMBRYO. 



epiblast, so it is only the epithelium of the former which comes from 

 the hypoblast. 



Growth in length of the Vertebrate Embryo. 



With reference to the formation and growth in length of the body of 

 the Vertebrate embryo two different views have been put forward, which 

 can be best explained by taking the Elasmobranch embryo as our type. One 

 of these views, generally held by embryologists and adopted in the previous 

 pages, is that the Elasmobranch embryo arises from a differentiation of the 

 edge of the blastoderm; which extends inwards from the edge for some little 

 distance. This differentiation is supposed to contain within itself the rudi- 

 ments of the whole of the embryo with the exception of the yolk-sack ; 

 and the hinder extremity of it, at the edge of the blastoderm, is regarded 

 as corresponding with the hind end of the body of the adult. The growth 

 in length takes place by a process of intussusception, and, till there are 

 formed the full number of mesoblastic somites, it is effected, as in Chpeto- 

 pods, by the continual addition of fresh somites between the last-formed 

 somite and the hind end of the body. 



A second and somewhat paradoxical view has been recently brought 

 into prominence by His and Rauber. This view has moreover since been 

 taken up by many embryologists, and has led to strange comparisons 

 between the formation of the mesoblastic plates of the Chsetopods and 

 the medullary folds of Vertebrata. According to this view the embryo 

 grows in length by the coalescence of the two halves of the thickened edges 

 of the blastoderm in the dorsal median line. The groove between the 

 coalescing edges is the medullary groove, which inci'eases in length by the 

 continued coalescence of fresh portions of the edge of the blastoderm. 



The following is His' own statement of his view: "I have shewn that 

 the embryo of Osseous Fishes grows together in length from two symmetri- 

 cally-placed structures in the thickened edge of the blastoderm. Only the 

 foremost end of the head and the hindermost end of the tail undergo no 

 concrescence, since they are formed out of that part of the edge of the 

 blastoderm which, together with the two lateral halves, completes the ring. 

 The whole edge of the blastoderm is used in the formation of the embryo." 



The edges of the blastoderm which meet to form the body of the embryo 

 are regarded as the blastopore, so that, on this view, the blastopore primi- 

 tively extends for the whole length of the dorsal side of the embryo, and 

 the groove between the coalesced lips becomes the medullary groove. 



It is not possible for me to enter at any great length into the arguments 

 used to support this position. 



They may be summarised as (1) The general appearance; i.e. that the 

 thickened edge of the blastoderm is continuous with the mediillary fold. 



(2) Certain measurements (His) which mainly appear to me to prove 

 that the growth takes place by the addition of fresh somites between that 

 last formed and the end of the body. 



(3) Some of the phenomena of double monsters (Rauber). 



None of these arguments appear to be very forcible, but as the view of 

 His and Rauber, if true, would certainly be important, I shall attempt 

 shortly to state the arguments against it, employing as my type the 

 Elasmobranchii, by the development of which, according to His, the view 



