of the Fishery Board for Scotland. 



277 



embryo, or in preparations of unsectioned embryos. The authors who have 

 described the origin of the heart from such specimens deal not with the 

 first appearance of the structures that form the heart, but only with stages 

 later than the very earliest. So Lereboullet's * description of the heart in 

 the trout, pike, and perch, as 'un corps cylindrique ou conique, solide, plein, 

 compose de cellules couche sous la tete,' refers to a stage somewhat subsequent 

 to the first appearance of the heart. Kupffer's f fig. 9 of Gasterosteus and 

 description of the sac, as ''Ein strumpf Jwnischer Korper, 1 applies only to the 

 already well-formed heart. Wenckebach's % careful investigation into the 

 origin of the heart in Belone, is based almost entirely on observations 

 made on the living embryo, and his definite mesoblastic band derives more 

 support from external views than from anything I can find in my sections. 

 Though I have not seen anything in the plaice to support the view which 

 he gives in fig. 7, yet I must agree with him and with Ziegler,§ that the 

 cells which go to form the heart are derived from the indifferent mesoblast 

 of the head. 



In the embryo, about the beginning of the eighth day, sections across 

 the region of the hind brain show the heart as a quadrangular cavity 

 (fig. 8, en.), bounded above by the lower layer of the fore-gut, below by the 

 yelk, and laterally by the splanchnic layers of the mesoblast approaching 

 from either side. The section of the tube is composed of a very thin 

 layer of tissue with nuclei here and there, and this tissue penetrates from 

 each of the four corners, above, between the ventral walls of the oesophagus 

 and the upper limbs of the splanchnic layers already mentioned, and below, 

 between the ventral limbs of the splanchnopleure and the ventral yelk. 

 This cavity is the true cavity of the heart, and the tissue of its walls 

 ultimately becomes the endocardial lining of the myocardium. As to the 

 origin of the endocardium, it is to be noted that it is seen to be continuous 

 between the ventral layer of splanchnopleure and the ventral wall of the 

 body, with the mass of mesoblast figured on the right side. This meso- 

 blast stretches from the branchial invagination (br.c.) to the fore-gut (f.g.), 

 and to the two layers of mesoblast approaching the median ventral line ; 

 and it can be traced to isolated cells lying underneath the ventral layer of 

 splanchnopleure — marked in the figure v).c. — which are in close relation 

 with the ventral prolongation of the endocardial tube. The cells marked 

 w.c. apparently represent the cells marked kz. (' wanderzellen ') by Ziegler 

 in figs. 30 et seq., and, with Ziegler, I believe that they do not arise 

 from nuclei of|the yelk. Hoffman's || view that the endothelium is ' Ein 

 product des entoderms,' and that the cells of the endothelium arise 

 1 Durch Proliferation des liter gelegenen Zellen des Entoderms des Para- 

 blastes,' is not supported by anything I have seen in sections of the 

 plaice, but the reverse is the case. Hoffman's figures (taf. ii. fig. 9, taf. 

 iii. fig. 4, and taf. iv. fig. 6), are too diagrammatic and symmetrical for 

 much reliance to be placed on them. 



This endocardial cavity soon becomes surrounded by the union of the 

 dorsal splanchnic plates {Seitenplatten of Ziegler, and Pericardialplatten 

 of Oellacher) above, and by the confluence of the ventral splanchnopleure 

 layers. In embryos of half a day older than fig. 8, the endocardial tube 



* £ Recherches d'Embryologie comparee sur le developperaent de la Truite,' &c. — 

 Ann. de Sc. Nat. , torn. xix. 



t ' Beobachtungen iiber die Entwicklmig der Knockenfische,' Archiv. f. Mikros. 

 Anat., Bd. iv. 



X ' Zur Entwicklungsgeschichte der Knockenfische,' Archiv. f. Mikros. Anat, 

 Bd. xxviii. 



§ 1 Die Entstehung des Blutes bei Knockenfischembryonen,' Archiv. f. Mikros. Anat., 

 Bd. xxx. 



II 1 Zur Ontogenie der Knockenfische,' Natuurk Verh. der Koninkl. Akad, van 

 Wetens. te Amsterdam, 1882. 



