192 PROFESSOR MARSHALL AND EDWARD J. BLES. 



Oellacher* commences his account of the development of the 

 heart in Bufo at a stage a little earlier than that shown in Plate 

 XIIL, Fig. 3. He describes and figures the whole of the cavity below 

 the pharynx, between hypoblast and mesoblast, as filled up by a solid 

 mass of cells, whose origin he was unable to determine, and from 

 some of which the endothelium of the heart is derived. His figures 

 suggest that what he describes as a solid mass of cells is really, in 

 great part at least, a coagulum ; and his observations did not begin, 

 at a sufficiently early stage to enable him to determine the origin 

 of the endothelium. 



Goettef describes the endothelial cells in Bombinator as forming 

 at first a loose discontinuous layer of cells derived from the hypo- 

 blast of the pharyngeal wall, but possibly including also cells derived 

 from the mesoblast of the splanchnopleure. 



Balfour! says, concerning the cardiac endothelium of Elasmo- 

 branchs : " The origin of this lining layer I could not certainly 

 determine, but its connection with the splanchnic mesoblast suggests 

 that it is probably a derivative of this." Concerning the Fowl, he 

 says : " I have been able to satisfy myself that the epithelioid lining 

 of the heart is derived from the splanchnic mesoblast. When the 

 cavity of the heart is being formed by the separation of the splanchnic 

 mesoblast from the hypoblast, a layer of the former remains close to 

 the hypoblast, but connected with the main mass of the splanchnic 

 mesoblast by protoplasmic processes. A second layer next becomes 

 split from the splanchnic mesoblast, connected with the first layer 

 by the above-mentioned protoplasmic processes. These two layers 

 form the epithelioid lining of the heart ; between them is the cavity 

 of the heart, which soon loses the protoplasmic trabecule which at 

 first traverse it." 



Eabl§ notices that in Salamandra and Triton a median longitu- 

 dinal groove is present in the floor of the pharynx, extending back- 



* Oellacher, "Ueber die erste Entwickelung des Herzens und der Pericardial* 

 oder Herzhohle bei Bufo cinereus : " "Arch. f. Mikr. Anatomie," vii., 1871, pp. 157- 

 165. 



t Goette, "Entwickelungsgeschichte der Unke," 1875. 



J Balfour, " Elasmobranch Fishes," 1878, p. 230, and " Comparative Embryology," 

 1881, vol. ii., p. 521. 



§'Rabl, " Ueber die Bildung des Herzens der Amphibien : " " Morphologisches Jahr- 

 buch," xii., 1887, pp. 252-274. 



