128 DEVELOPMENT OF THE BLOOD. 



cal with the formative, vitelline, or embryonic cells of which all the 

 structures of the embryo are originally composed ; that they are, in fact, 

 the central cells of the solid mass of which the heart and large blood- 

 vessels at first consist. A difference of opinion, however, still exists 

 with respect to the mode in which these original cells are converted 

 into the characteristic corpuscles subsequently found in the blood. 



According to Vogt, from observations made on the larva of the toad,f 

 (Alytes obstetricans,) and on the embryo of the salmon (Coregonus 

 palseaj), the cell-wall of each original cell gradually disappears, and the 

 liberated nucleus, in which a secondary nucleus is subsequently formed, 

 becomes the true nucleated blood-corpuscle. The circumstances which he 

 urges in favour of this view are, first, the close correspondence in size 

 between the nuclei of the original cells and the true corpuscles of the 

 blood ; and, secondly, the non-existence of a nucleus at first in the small 

 corpuscles, and its appearance subsequently. Against this view it is 

 objected, by MM. Prevost and Lebert, from observations also made on 

 Batrachians, that there is by no means so close a resemblance in size 

 between the nuclei of the primitive cells and the corpuscles of the blood, 

 as stated by Vogt, but that the size of the latter more nearly corresponds 

 with that of the cells themselves : and that, contrary to Vogt's statement, 

 a nucleus may be detected in the blood-corpuscle in all the phases of 

 its evolution. According to these observers, the blood-corpuscles result 

 from a direct transformation of the cells themselves, which assume an 

 ellipsoid instead of their previously round form, become flattened, lose 

 their granular matter, and acquire coloured contents. Kolliker, also, is 

 opposed to the account given by Vogt, and is of opinion with Prevost 

 and Lebert, that in Batrachians, and also in Mammalia, the embryonic 

 cells themselves are directly transformed into the true blood- corpuscles. 

 Like Prevost and Lebert, he also was unable to find non-nucleated 

 corpuscles in the blood of larval frogs, and of the earliest embryos of 

 Mammalia. A similar view to that of the last three named observers 

 concerning the direct origin of the earliest blood-corpuscles from the em- 

 bryonic cells, appears to be entertained also by most other physiologists^ 



The conversion of embryonic cells into true blood-corpuscles, in what- 

 ever way effected, is probably completed very shortly after the formation 

 of a cavity in the heart and in the large blood-vessels in connection with 



* Entwickelungs-geschichte der Geburtshelferkrb'te, p. 70. 



t In Agassiz's Hist. Nat. des Poissons d'eau douce, tome i. p. 203. 



$ An. des Sc. Nat. 1844, p. 212. An des Sciences. Nat. 1846, p. 43. 



|| Mr. Macleod (London and Edinb. Monthly Journ. of Med. Sc. Sept. 1842,) is of 

 opinion, however, that in the chick the earliest blood corpuscles are developed from minute 

 dark spherical granules of which alone the blood at first is composed. He believes that each 

 of these possesses the power of enlarging, and being developed into a circular nucleated cell, 

 which subsequently flattens, assumes colouring matter, grows oval, and thus becomes a true 

 red blood-corpuscle. 



