232 s. HATTA; FORMATION ON THE 



the heart, and being slightly oblique, shows the two cavities in it ; in 

 the next section after it the passage can be seen. In this stao-e 

 all the mesocardial Suspensorium atrophies and the cardiac tube is set 

 free through its whole lengtli. 



In a little more advanced stage the pronephric tubules have 

 become coiled many times and the ventral invagination of the 

 liver has grown anteriorly and pushed aside the hinder part of the 

 heart (Fig. 16). 



From the foregoing observations it is clear that in Petr(3mvzon 

 the mesencliymatous cells, between the two primary layers at 

 the point on the ventral median line where the head-f )ld joins the 

 larger posterior yolk-mass is transformed into the endothelium of 

 the heart. I am, therefore, unable to accept the statement of Shipley 

 (18) that this is derived from the splitting of the splanchnic plates of 

 the mesoblast. He says that (loco. cit. p. 17) : " from the first 

 appearance of the heart in the ventral mesentery its walls have 

 been double ; the splanchnopleure h;iving split into two layers ; 

 of these the outer is at first nuich the thicker consisting of cubical 

 cells ; the inner layer is composed of comparatively flattened cells." 

 It is probable tliat Shipley overlooked the earlier stages ; for his Fig. 

 25, in which the heart is said to appear first, shows a stage already 

 quite advanced corresponding with my Figs. () and 9. 



The endothelium of the heart is stated by C. Hoffm;uin (Î.)) and 

 by J. Riickert (14) to arise from the hypoblast in Selachian embryos. 

 C. Rabl (13) suggests the same in his account of Urode! Amphibia, 

 while F. Schwink (17) thinks that it comes out, not from the hypoblast 

 already formed, but from the yolk-mass, in both Urodela and Anura. 

 J. Oellacher (12) derives it from the mesoblast in Batrachia. Paul 

 Meyer (11) and F. Balfour (1) ascribe it to the same source in Selachia. 

 A. Götte believes that it is in Petromyzon (6) a derivative of the 



