COMPARISON OF THE GERMINAL LAYERS. 307 



formation of the mesoblastic plates of the Chaetopods 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 medul- 

 lary groove, which increases 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 symmetrically- 

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

 most end of the head and the hindermost end of the tail undergo no concre- 

 scence, 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 (i) The general appearance ; i.e. that the 

 thickened edge of the blastoderm is continuous with the medullary 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 Elasmobran- 

 chii, by the development of which, according to His, the view 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 be- 

 coming 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 is continuous with the adjoin- 

 ing epiblast of the embryonic rim, the latter structure must be continuous 

 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 concrescence 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 



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