540 REPORT OF C0MM:ISSI0NER OF FISH AND FISHERIES. [86J 



In Siphostoma a subintestinal vein passes clown bebind the yelksack 

 and traverses its ventral surface in the middle line to empty into the 

 heart in front, which does not have its venous end prolonged but simply 

 opens to this median vessel in a ventral direction. 



The embryos of Fundulus have the heart somewhat prolonged over the 

 yelk anteriorly. 



In Gamhusia, a diffuse, superficial, vitelline capillary system arises 

 from a very short subintestinal vein and lateral venous trunks which are 

 probably Cuvierian, but which are also assisted by the hepatic vein on 

 one side. The capillaries so arising converge at the anterior end of the 

 yelk, where the venous end of the heart is prolonged downwards between 

 the epiblastic covering of the yelk and the yelk hypoblast. 



In the cod, when the blood-vessels are developed, thirty days after 

 hatching, the venous sinus opens upwards and backwards and receives 

 three sets of vessels, viz, lateral and ventral intestinal and the cardinal 

 veins, the latter by way of the Cuvierian ducts. 



In Cottus a pair of anteriorly divergent veins, lying on the ventral 

 face of the yelk-sack, pass upwards and forwards to empty into the 

 venous sinus, just below where the cardinals debouch. 



Alosa, Cyhium, Parephippus, Elacate^ Osnierus, and Fomolohus do not 

 have a vitelline circulation at all, and here the heart soonest acquires 

 its adult position, as in the cod. But in all of these forms it is in the 

 highest degree probable that the heart opens directly into the segmen- 

 tation or body cavity, as I have demonstrated in Alosa and Pomolobus. 

 The mode of absorption of the yelk in these forms also becomes clear 

 on the grounds already stated in my paper on that subject, namely, 

 by direct gemmation of corpuscles from the yelk hypoblast into the 

 segmentation cavity, from whence they are taken up into the circula- 

 tion by the heart. 



28. — Development of the circulation and the function op 

 the yelk hypoblast. 



The various physiological adaptations of the circulatory system, if 

 we may so speak, which we have described in the preceding chapter, 

 show us clearly that one and the same functiom may be performed by 

 the profound, almost radical, modification of the system of organs 

 which is concerned in its manifestation. In no set of organs within a 

 restricted grouj) of types do we find any instance which presents more 

 striking variations than those observable in the arrangement of the 

 vessels upon the yelks of different species of embryo Teleosts. To 

 trace the course of the vessels themselves in the different forms to be 

 described is no easy task ; this will therefore not be attemi)tcd with the 

 less important ones, but only with the larger vascular trunks, which 

 are also the first to be developed. 



The development of the vessels themselves is so important for us to 



