DEVELOPMENT OF ELASMOBRANCH FISHES. 121 



No fact in my investigations comes out more clearly than that 

 a great part of the voluntary muscular system is formed from 

 the splanchnic layer of the mesoblast, yet this fact presents 

 a most serious difficulty to the view that the somatic and 

 splanchnic layers of the mesoblast in Vertebrates are respectively 

 derived from the epiblast and hypoblast. 



In spite, therefore, of general a 'priori considerations of 

 a very convincing kind which tell in favour of the double 

 origin of the mesoblast, this view is supported by so few 

 objective facts, and there exists so powerful an array of facts 

 against it, that at present, at least, it seems impossible to main- 

 tain it. The full strength of the facts against it will appear 

 more fully in a review of the present state of our knowledo-e as 

 to the development of the mesoblast in the different groups. 



To this I now pass. 



In a paper on the " Early stages of development in Verte- 

 brates^" a short resume was given of the development of the 

 mesoblast throughout the animal kingdom, which it may be 

 worth while repeating here with a few additions. So far as we 

 know at present, the mesoblast is derived from the hypoblast 

 in the following groups: 



Echinoderms(Hensen, Agassiz, Metschnikofif, Selenka, Gotte), 

 Nematodes (Butschli), Sagitta (Kowalevsky, Butschli), Lum- 

 bricus and probably other Annelids (Kowalevsky), Brachiopoda 

 (Kowalevsky), Crustaceans (Bobretzky), Insects (Kowalevsky, 

 Ulianin, Dohrn), Myriapods (Metschnikoff), Tunicates (Kowa- 

 levsky, Kuppfer), Petromyzon (Owsjanikoff), Osseous fishes 

 (Oellacher, Gotte, Kowalevsky), Elasmobranchs (Self), Amphi- 

 bians (Remak, Strieker, Gotte). 



The list includes members from the greater number of the 

 groups of the animal kingdom ; the most striking omis- 

 sions being the Coelenterates, Mollusks, and the Amniotic Ver- 

 tebrates. The absence of the Coelenterates has been already 

 explained, and my grounds for regarding the Amniotic Vertebrates 

 as apparent rather than real exceptions have also been pointed 

 out. The Mollusks, however, remain as a large group, in which 

 we as yet know very little as to the formation of the mesoblast. 



^ Q,uaYt. JL of Micros. Science, July, 1875. 



