DEVELOPMENT OF ELASMOBRA^X'H FISHES. 119 



blast with absolute confidence from the hypoblast, while others 

 do so with equal confidence from the epiblast, he concludes that 

 it is really derived from both these layers. 



(2) A second argument is founded on the supposed deriva- 

 tion of the mesoblast in Amphioxus from both epiblast and 

 hypoblast. Ko^valevsky's account (on w^hich apparently Prof. 

 Haeckel's^ statements are based) appears to me, however, too 

 vague, and his observations too imperfect, for much confidence 

 to be placed in his statements on this head. It does not indeed 

 appear to me that the formation of the layers in Amphioxus, till 

 better known, can be used as an argument for any special view 

 about this question. 



(3) Professor Haeckel's own observations on the develop- 

 ment of Osseous fish form a third argument in support of his 

 views. These observations do not, however, accord with those of 

 the majority of investigators, and not having been made by 

 means of sections, require further confirmation before they can 

 be definitely accepted. 



(4) A fourth argument rests on the fact that the various 

 embryonic layers fuse together to form the primitive streak or 

 axis-cord in higher vertebrates. This he thinks proves that the 

 mesoblast is derived from both the primitive layers. The primi- 

 tive streak has, however, according to my views, quite another 

 significance to that attributed to it by Professor HaeckeP; but 

 in any case Professor Kolliker's researches, and on this point my 

 own observations accord w^ith his, appear to me to prove that the 

 fusion which there takes place is only capable of being used as 

 an argument in favour of an epiblastic origin of the mesoblast, 

 and not of its derivation from both epiblast and hypoblast. 



The objective arguments in favour of Professor Haeckel's 

 view^s are not very conclusive, and he himself does not deny 

 that the mesoblast as a rule apparently arises as a single and 

 undivided mass from one of the two primary layers, and only 

 subsequently becomes split into somatic and splanchnic strata. 

 This original fusion and subsequent splitting of the mesoblast 



1 Vide Anthropogenie, p. 197. 



2 Vide Self, Development of Elasmobranch Fishes, Journal of Anat. ar^d 

 Phys. Vol. X. note on p. 682, and also Eeview of Professor Kolliker's Entwick- 

 lungsgeschichte des Menschen u. d. hoheren Thiere, Journal of Anat. and Phys. 

 Vol. X. 



