[85] ElilBRTOGKAPHY OF OSSEOUS FISHES. 539 



and systole of the ventricle. These phenomena are well known to physi- 

 ologists, and some of their effects are matters of every day-class demon- 

 stration, upon man and the higher animals, by means of the sphymo- 

 graph; in other words, the fish embryo has a pulse. 



It has already been remarked that wide differences of cardiac develop- 

 ment occur in different genera and families of osseous fishes. Take the 

 case of Idus melmioius^ the golden ide; in the embryo of tiiis species the 

 venous end of the heart grows down between the front end of the yelk 

 and its epiblastic covering until the heart itself lies ventrad of the ante- 

 rior part of the yelk-mass. The Cuvierian ducts, which collect the blood 

 from the cardinal veins, actually pass around the front end of the yelk 

 on either side, and join the venous sinus below it; besides these venous 

 vascular arches there is no circulation over the yelk-sack in this species. 



In Tylosurus the venous end of the heart is prolonged in front of the 

 head of the embryo into an annular vessel which traverses the entire 

 circumference of the yelk in a plane coinciding with the axis of the body. 

 Later two vessels arise from the cardinal veins which carry the blood 

 from the body over the yelk back to the outlying venous end of the 

 heart. Then below the head a huge pericardiac space or chamber is 

 gradually formed, which is roofed over entirely by the epiblastic cov- 

 ering of the yelk-sack into which the heart depends, having been dis- 

 proportionally elongated in consequence. Its venous end is fixed to the 

 extreme lower part of the huge heart chamber where it is continuous with 

 the yelk hypoblast or blood-generating layer which overlies the yelk. 

 Here at its point of attachment the three vitelline veins join the heart 

 and pour their contents into it. The remarkable abundance of blood 

 corpuscles in the heart cavity and their origin has already been described 

 in my paper on this species, so that I will not here repeat what has al- 

 ready been well enough elaborated elsewhere. 



In Apeltes the venous end of the heart is pushed out from the right 

 side of the body and is at first joined to an asymmetrical system of 

 vitelline vessels, which at a later stage become quite symmetrically ar- 

 ranged. 



In Salmo the heart is never prolonged outwards anteriorly or laterally 

 in the embryo, as in the foregoing species. The vitelline system of ves- 

 sels develop somewhat asymmetrically, and the great venous vitelline 

 trunk does not lie in the middle line but somewhat to the left side. A 

 part of the blood which passes through the vitelline capillaries passes 

 through the liver, and there are no greatly developed representatives 

 of the Cuvierian ducts which traverse the yelk-sack, as in Idus and Tylo- 

 surus. 



In the young gold-fish, Carassius, the Cuvierian ducts embrace the 

 anterior extremity of the yelk as in Idus, in order to reach the heart, 

 which is ventrad of the yelk in jjosition. 



