212 H. D. Senior. 



Introduction. 



A Brief Description of the Heart and Great Venous Channels 

 of a Recently Hatched Shad. 



In examining a recently hatched specimen of shad, it will be found 

 that the heart (Fig. 1) is apparently widely open at the venous end. 

 The atrium is separated from the ventricle in the usual way, but, 

 on tracing the atrial wall back to where the sinus venosus should 

 be, no sinus venosus, as such, is found to exist. The heart wall 

 immediately succeeding the somewhat constricted venous end of the 

 atrium represents the anterior wall of the future sinus venosus. 

 This flares out abruptly, and its circumference, having reached the 

 body-wall, is reflected forward upon the latter as the parietal peri- 

 cardium. These relations are shown in Fig. 1 ; the general arrange- 

 ment of the circulation, at this period, is diagrammatically indicated 

 in Fig. 2. i J 



The relations here are obviously peculiar ; the peculiarity, how- 

 ever, does not consist in the continuity of the myo-epicardium with 

 the parietal pericardium. Inasmuch as both the myo-epicardium 

 and the parietal pericardium are developed from the mesothelium of 

 the lateral plates in all vertebrates, this continuity is invariable. The 

 peculiar feature in connection with the heart, in its present state, 

 is that the anterior pole of the yolk in a sense replaces the posterior 

 wall of the sinus venosus. Since the yolk is entirely naked, the 

 vascular system, closed though it is, is not completely lined by 

 vascular endothelium, as is very commonly the case in vertebrate 

 embryos at a comparatively early stage of development. 



The venous blood is returned to the heart through four veins, the 

 jugulars and cardinals ; also by means of a blood-sinus situate dorsal 

 to the yolk, which may be called the supra vitelline blood-sinus (or, 

 for the sake of brevity, the supravitelline sinus). The supra vitelline 

 sinus is formed in the following way : The peritoneum has a ventral 

 attachment on either side to the dorsal surface of the yolk; the 

 peritoneal attachments involve almost the entire longitudinal extent 

 of the yolk. Between the lines of attachment there is enclosed an 

 arched tunnel, of which the floor is formed by the surface of the yolk, 



