THE HEART. 461 



front the cavity of the heart is bounded by the approximation 

 of the splanchnic mesoblast to the wall of the throat, and be- 

 hind by the stalk connecting the alimentary canal with the 

 yolk-sack. 



As development proceeds the ventral wall of the heart be- 

 comes bent inwards on each side on a level with the wall of the 

 gut (Plate ii, fig. 4), and eventually becomes so folded in as 

 to form for the heart a complete muscular wall of splanchnic 

 mesoblast. The growth inwards of the mesoblast to form the 

 dorsal wall of the heart does not, as might be expected, begin in 

 front and proceed backwards, but commences behind and is 

 gradually carried forwards. 



From the above account it is clear that I have failed to 

 find in Elasmobranchs any traces of two distinct cavities co- 

 alescing to form the heart, such as have been recently de- 

 scribed in Mammals and Birds ; and this, as well as the other 

 features of the formation of the heart in Elasmobranchs, are in 

 very close accordance with the careful description given by 

 Gotte 1 of the formation of the heart in Bombinator. The di- 

 vergence which appears to be indicated in the formation of so 

 important an organ as the heart between Pisces and Amphi- 

 bians on the one hand, and Aves and Mammalia on the other, 

 is certainly startling, and demands a careful scrutiny. The 

 most complete observations on the double formation of the 

 heart in Mammalia have been made by Hensen, Gotte and 

 Kolliker. These observations lead to the conclusion (i) that 

 the heart arises as two independent splits between the splanchnic 

 mesoblast and the hypoblast, each with an epithelioicl (endo- 

 thelial) lining. (2) That the heart is first formed at a period 

 ivlien the folding in of the splanclmopleure to form the throat has 



protoplasmic processes. A second layer next becomes split from the splanchnic 

 mesoblast, connected with the first layer by the above-mentioned protoplasmic pro- 

 cesses. These two layers form the epithelioid lining of the heart ; between them is 

 the cavity of the heart, which soon loses the protoplasmic trabeculae which at first 

 traverse it. 



1 Bischoff has recently stated, Historisch-kritische Bemerkungen u.d. Entwickeiuug 

 d. Saiigethu-reier, that Gotte has found a double formation of the heart in Bombinator. 

 It may seem bold to question the accuracy of BischofF's interpretation of writings in 

 his own language, but I have certainly failed to gather this either from Dr Gotte' s text 

 or figures. 



