40 M. Ph. Owsjannikow on the 



margin a special layer of flat cells. This internal layer of 

 cells separates from the external one ; and in the transverse 

 sections we have before us two tubes lying one within the 

 other. The internal one becomes the endothelial coating of 

 the heart — that is, the endocardium. The second, somewhat 

 stouter tube becomes the muscular tissue of the heart, which 

 on the outside is likewise clothed with endothelial cells. The 

 heart in Petromyzon arises from the folds of the intestinal 

 fibrillar plates, which alone furnish the material for all its 

 constituent parts. I would especially point out that the 

 elements of the intestine take no share whatever in the forma- 

 tion of the heart. A communication is established between 

 the cardiac cavity and those lymph-chambers which we 

 designated above as the commencement of the heart, and 

 which have transformed themselves into the veins. At this 

 period the veins have no walls of their own, and are in com- 

 munication with many canals and lacuna?. 



Though the literature of the subject will not be fully con- 

 sidered until my detailed and illustrated paper is printed, 

 I cannot here refrain from mentioning that A. fehipley's obser- 

 vations with reference to the heart also are highly valuable. 

 His figures (figs. 24 & 26) are faithful representations, albeit 

 somewhat diagrammatic. At the time of the earliest formation 

 of the heart the endothelium on the intestinal fibrillar plate 

 cannot be seen with the distinctness with which it is shown 

 in the figure. 



Since the formation of the heart is one of the most difficult 

 processes which embryology has to study, I would like to add 

 a few more words to the results above described. According 

 to an observation by P. Meyer, as communicated in his article 

 " Ueber die Entwickelung des Herzens und der grossen Ge- 

 fiissstamme bei den Selachiern," the endocardium arises in a 

 manner different from that which we have explained. This 

 author states that when the intestine closes the two lateral 

 veins apply themselves together beneath it, and unite into one 

 vessel, to provide the heart with a single unpaired tube — an 

 " endothelial saccule." The figures (Tab. 12. fig. 2, and 

 others) which P. Meyer has given us entirely support his 

 view ; and in the embryology of the lamprey also we actually 

 find a series of preparations which appear to confirm it. For 

 it is an exceedingly striking fact that the endothelial saccule 

 of the heart often lies at a great distance from the myocardium, 

 as if it has arisen independently and has been in no way con- 

 nected with the lateral plates. Moreover, we find the cardiac 

 cavity bounded by the lateral plates, without its being possible 

 to recognize distinctly the endothelium on their inner surface. 



