DERIVATION OF THE MESOBLAST. 339 



it is very difficult to understand how there could be formed from 

 it a part of the voluntary muscular system of the body in- 

 distinguishably fused with part of the muscular system derived 

 from the somatopleure. No fact in my investigations comes 

 out more clearly than that a great part of the voluntary mus- 

 cular system is formed from the splanchnic layer of the meso- 

 blast, 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 maintain it. 

 The full strength of the facts against it will appear more fully 

 in a review of the present state of our knowledge 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 Ver- 

 tebrates 1 " 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, Metschnikoff, Selenka, Gotte), 

 Nematodes (Butschli), Sagitta (Kowalevsky, Biitschli), 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 omissions 

 being the Ccelenterates, Mollusks, and the Amniotic Vertebrates. 

 The absence of the Ccelenterates has been already explained 

 and my grounds for regarding the Amniotic Vertebrates as 



1 Quart. Jl. of Micros. Science, July, 1875. [This Edition, No. vi.] 



