322 DEVELOPMENT OF ELASMOBRANCH FISHES. 



in the way of supposing that they can have originated de novo, 

 and not by the modification of some pre-existing organ, and 

 suggests that the limbs are modified gill-arches ; a view similar 

 to which has been hinted at by Professor Gegenbaur 1 . 



Dr Dohrn has not as yet given the grounds for his determina- 

 tion, so that any judgment on his views is premature. 



None of my observations on Elasmobranchs lends any sup- 

 port to these views ; but perhaps, while regarding the limbs as 

 the remains of a continuous fin, it might be permissible to 

 suppose that the pelvic and thoracic girdles are altered remnants 

 of the skeletal parts of some of the gill-arches which have 

 vanished in existing Vertebrates. 



The absence of limbs in the Marsipobranchii and Amphioxus, 

 for reasons already insisted upon by Dr Dohrn 2 , cannot be used 

 as an argument against limbs having existed in still more 

 primitive Vertebrates. 



Though it does not seem probable that a dorsal and ventral 

 fin can have existed contemporaneously with lateral fins (at 

 least not as continuous fins), yet, judging from such forms as 

 the Rays, there is no reason why small balancing dorsal and 

 caudal fins should not have co-existed with fully developed 

 lateral fins. 



Mesoblast. GK. 



The mesoblast in stage F forms two independent lateral 

 plates, each with a splanchnic and somatic layer, and divided, 

 as before explained, into a vertebral portion and a parietal 

 portion. At their peripheral edge these plates are continuous 

 with the general mesoblastic tissue of the non-embryonic part of 

 the blastoderm ; except in the free parts of the embryo, where 

 they are necessarily separated from the mesoblast of the yolk- 

 sac, and form completely independent lateral masses of cells. 



During the stages G and H, the two layers of which the 

 mesoblast is composed cease to be in contact, and leave be- 

 tween them a space which constitutes the commencement of the 

 body-cavity (PI. 10, fig. i). From the very first this cavity is 

 more or less clearly divided into two distinct parts ; one of them 



1 Grundriss d. Vergleichenden Anat. p. 494. 

 2 Loc. tit. 



