114 GENERAL CONSIDERATIONS. 



mesoblast in Elasmobranchs and in other Vertebrates, in order 

 to distinguish as far as possible the primitive and the secondary- 

 characters present in the various groups. 



Though the Mammals are to be looked on as the most 

 differentiated group amongst the Vertebrates, yet in their 

 embryonic history they retain many very primitive features, 

 and, as has been recently shewn by Hensen^ present numerous, 

 remarkable approximations to the Elasmobranchs. We find ac- 

 cordingly^ that the primitive lateral plates of mesoblast undergo, 

 nearly the same changes in these two groups. In Mammals 

 there is at first a continuous cavity extending through both 

 the parietal and vertebral portions of each plate, and dividing ^ 

 the plates into a somatic and a splanchnic layer : this cavity is 

 the primitive body-cavity. The vertebral portion of each plate 

 with its contained cavity then becomes divided off from the 

 parietal. The later development of these parts is not accurately 

 known, but it seems that the outer portion of each vertebral 

 plate, composed of two layers (somatic and splanchnic) en- 

 closing between them a remnant of the primitive body-cavity, 

 becomes separated off as a muscle-plate. The remainder 

 forms a vertebral rudiment, &c. Thus the extension of the 

 body-cavity into the vertebral portion of the mesoblast, and the 

 constriction of the vertebral portion of the cavity from the re- 

 mainder, are as distinctive features of Mammals as they are of 

 the Elasmobranchs. 



In Birds ^ the horizontal splitting of the mesoblast into 

 somatic and splanchnic layers appears, as in Mammals, to extend 

 at first to the summit of the protovertebrse, but these bodies 

 become so early separated from the parietal plates that this 

 fact has usually been overlooked or denied ; but even on the 

 second day of incubation the outer layer of the protovertebrse is 

 continuous with the somatic layer of the lateral plates, and 

 the inner layer and kernel of the protovertebrai with the 

 splanchnic layer of the lateral plates \ After the isolation 



1 Zeitschrift f. Anat. EntwicJclungsgeschichte, Vol. i. 



2 Hensen loc. cit. 



3 For the history of protovertebrae and muscle-plates in Birds, vide Ele- 

 ments of Embryology, Foster and Balfour, The statement there made that the 

 horizontal splitting of the mesoblast does not extend to the summit of the 

 vertebral plate, must however be regarded as doubtful. 



* Vide ElemeiUs of Embryology, p. 56. 



