DEVELOPMENT OF ELASMOBRANCH FISHES. 281 



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 x. fig. 4), and eventually becomes so folded 

 in as to form for the heart a complete muscular wall of splanch- 

 nic mesoblast. The growth inwards of the mesoblast to form 

 the dorsal w^all 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 

 coalescing 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^ 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 Amphibians 

 on the one hand, and Aves and. Mammalia on the other, is cer- 

 tainly startling, and demands a careful scrutiny. The most com- 

 plete observations on the double formation of the heart in 

 Mammalia have been made by Hensen, Gotte and Kolliker. 

 These observations lead to the conclusion (1) that the heart 

 arises as two independent splits between the splanchnic meso- 

 blast and the hypoblast, each with an epithelioid (endothelial) 

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

 folding in of the sijlanchnopleure to form the throat has not com- 

 menced, and luhen therefore it woidd he impossible for it to he 

 formed as a single tuhe. 



In Birds almost every investigator since von Baer has 

 detected more or less clearly the coalescence of two halves to 

 form the unpaired hearth Most investigators have however 

 believed that there was from the first an unpaired anterior 

 section of the heart, and that only the posterior part was 

 formed by the coalescence of two lateral halves. Professor 



1 Bischoff has recently stated, Historisch-kritische Bemerkungen ii. d. Ent- 

 icickelung d. Sdiigethiereier, that Gotte has foiind 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. 



2 Vide Elements of Embryology, Foster and Balfour, pp. 64—66. 



