of the Fishery Board for Scotland. 



279 



Paul Mayer,* following Rabl,f says, 1 1st die aussere Wand des 

 ' Herzens nur ein Accessorium und das Endocardium das eigentliche 

 1 Herzrohr.' From what has been said above, it follows that the Teleosteans 

 agree with the Selachians and Amphibians, of which those authors re- 

 spectively treat, in the endocardium being the heart proper, and the 

 myocardium being an accessory portion. In my sections of embryos of 

 different ages the endocardial lining is always present within the myocar- 

 dium, so that no support is afforded the view that it is a product of the 

 myocardium. % 



Oellacher was the first to point out the origin of the heart (' Herzanlage ') 

 from the lateral mesodermic plates of the head, but his conclusions as to 

 the origin of the endothelium from the mesoblast are not so completely 

 demonstrated as is the similar result at which Ziegler has arrived in 

 sections of younger embryos. Zeigler§sums up his statements on this 

 subject thus : — ' Das letztere {Endothel) mitsammt einer Anzahl von 

 1 Wanderzellen entstammt einer Gruppe von Mesodermzellen ; diese Zellen 

 \ sind in continuirUclier Forsetzung des Mesoderms des Kopfes.' Henne- 

 guy || concludes, ' Le result at de mes observations est absolument semblable 

 \ a celai de ces deux auteurs,' viz., Oellacher and Zeigler, but his figures, 

 Nos. 104 and 114, as well as his description, shows the heart of the trout 

 to have a double origin. According to him, an endothelial tube is pro- 

 duced on each side under the splanchnopleure layers, and these two tubes 

 ultimately fuse together. In the plaice only one, not two tubes, are 

 formed. With this difference, my results are in harmony with those of 

 Henneguy. Hoffman's H 'Entoderm des Parablastes,' from which he 

 derives the endothelium of the heart, is, I believe with Ziegler, only ' part 

 of the endothelium of the heart found under the splanchnopleure.' To 

 derive this from parablast is only a begging of the question, for the origin 

 of the cells, which he figures and calls entoderm-parablast, so far as I can 

 make out, has not been determined by him. 



The heart, therefore, is primarily an endothelial tube derived from the 

 lateral plate of mesoblast of the head, and the myocardium is formed by 

 the splanchnopleure which bounds the endocardial tube laterally. The 

 pericardium is two lateral cavities within the dorsal and ventral layers of 

 splanchnopleure of either side, and on the meeting together of the dorsal 

 layers of opposite sides and the union of the ventral layers, the cavities 

 fuse, and the pericardium becomes impaired. 



The renal organs appear shortly after the endothelial tube of the heart 

 is formed, and about a day before the ventral wall of the mesenteron 

 shows the thickening which gives rise to the liver. In the plaice, towards 

 the close of the eighth day, the solid condition of the renal organ is trace- 

 able through several sections, about the middle of the embryo. Figures 

 18, 19, and 20 indicate the origin of the organ from the intermediate cell- 

 mass. The pronephric duct seems to arise as a fold directed laterally and 

 dorsally. In the most anterior section (fig. 20), the intermediate cell-mass 

 of mesoblast seems to become nearly divided into two portions towards 

 the middle. The lower portion (fig. 20, pm.), between the gut and 

 the larger part of the somatoblast, is continuous externally with 

 the ventral surface of the latter, and is composed of cells having 

 large nuclei ; it abuts on the gut on the inner and upper face, and 

 is separated from it ventrally by the small portion of the coeloni. 



* 'Ueber die Entwicklung des Herzens, &c, bei den Selachien,' Mitth. Zool. St., 

 Naples, Bd. vii. Hit. ii. 



t ' Ueber die Bildung- des Herzens der Amphibien,' Morp. Jahrb., Bd. xii. 



J M'Intosh and Prince derive the endocardium from the walls of the myocardium. 

 Trans. Royal Society, Edin., 1890. § Loc. ext., p. 622. 



|| 'Recherches sur le dcveloppement des poissons osseux,' Journal de VAnatomie, 

 vol. xxiv. N.S. IT Loc. cit. 



