DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 777 



sections of our forms. Certainly it is not formed, as has been suggested for the chick, 

 by wandering corpuscles from the area vasculosa, which, finding access to the heart, cling 

 to its walls as a loose lining, for in Teleosteans this endocardium is present before the 

 haemal circulation is in action. Hoffman's figures (No. 69a, Taf. iii. fig. 9, Taf. iv. fig. 6) 

 do not represent the primary condition in our forms, for the heart usually pushes down 

 before it a delicate stratum of hypoblastic cells (hyp, PL VIII. fig. 11; PL XL fig. 2) ; 

 but this limiting ventral layer apparently becomes obliterated anteriorly, and the 

 pericardial chamber is open to the subembryonic space, which is undoubtedly the persist- 

 ing germinal cavity. The vermiform outline (h, PL VIII. fig. 3 ; PL XII. fig. 4) soon 

 undergoes modification, and the posterior end becomes expanded, while the anterior and 

 upper ventricular portion remains narrow (h, PL XIV. fig. 1). Thus the simple cardiac 

 tube becomes cone-shaped, the apex of the cone being continuous with the sub-cesophageal 

 mesoblast (nies, PL VIII. fig. 11), while the lower anterior end is comparatively free, 

 though not perfectly so, as a thin mesoblastic membrane (PL VIII. fig. 5) continuous 

 with the free edge of the auricle separates the myocardium from the exterior, and a space 

 is formed — the pericardial chamber (pd) around the heart. 



By its mode of formation as a downward growth the heart has at first a somewhat 

 vertical position ; but with its increase in length it extends further and further forward 

 beneath the head, and moreover it becomes flexed to the right (PL VIII. figs. 2, 9). The 

 anterior position of the heart at this time is quite characteristic of the early embryo. 

 The before-mentioned delicate pericardial walls are involved in the rhythmic movements 

 of the organ, and sway to and fro with each systole and diastole. 



The splanchnic mesoblast, out of which the heart and pericardium are formed, has 

 relations similar to the splanchnopleuric prolongation in the region of the trunk proper, 

 — the pericardial cavity surrounding the heart just as the cceloma encloses the abdominal 

 viscera, — the view that the former is merely a part separated off from the latter by the 

 posterior (pericardial) septum being strikingly supported by the condition in the 

 Cyclostomes, in which an intercommunication of pericardium and body-cavity persists 

 throughout life. 



The first change in the position of the slightly curved cylindrical heart (PL VIII. 

 fig. 5) results in its assuming an L-shaped form (as in PL IX. fig. 1, h), the small arterial 

 end (ventricle, ven) still occupying the median position, while the auricular end (aur) is 

 turned at right angles. In the figure before referred to, the flexure is still more apparent ; 

 while in fig. 3, PL XII., the auricle, previously directed to the front (PL VIII. figs. 3, 6, 8), 

 is now posterior (see also PL VIII. fig. 9), the flexure continuing to increase as development 

 proceeds. Thus the relations of the auricle and ventricle are reversed, and the latter, 

 which is now anterior, becomes bulbous (ven, PL VIII. fig. 7), and distinctly marked off 

 by a constriction ; while the auricle (au) itself is separated by a cincture into auricle 

 proper and sinus venosus (sv). The blind continuation of the ventricle into the sub- 

 pharyngeal mesoblast (mes, PL VIII. fig. 11) above is really the rudimentary bulbus 

 arteriosus, so that the four parts may be distinguished, as Ryder pointed out (No. 141, 



VOL. XXXV. PART III. (NO. 19). 6 G 



