TELEOSTEI. 8 1 



The most remarkable feature in which the young of a large number of 

 Teleostei differ from the adults is the possession of provisional spines, very 

 often formed as osseous spinous projections the spaces between which 

 become filled up in the adult. These processes are probably, as suggested 

 by Giinther, secondary developments acquired, like the Zooea spines of 

 larval Crustaceans, for purposes of defence. 



The yolk-sack varies greatly in size in the different types of 

 Teleostei. 



According as it is enclosed within the body-wall, or forms a distinct 

 ventral appendage, it is spoken of by Von Baer as an internal or external 

 yolk-sack. By Von Baer the yolk-sack is stated to remain in communication 

 with the intestine immediately behind the liver, while Lereboullet states that 

 there is a vitelline pedicle opening between the stomach and the liver which 

 persists till the absorption of the yolk-sack. My own observations do not 

 fully confirm either of these statements for the Salmon and Trout. So far 

 as I have been able to make out, all communication between the yolk-sack 

 and the alimentary tract is completely obliterated very early. In the Trout 

 the communication between the two is shut off before hatching, and in the 

 just-hatched Salmon I can find no trace of any vitelline pedicle. The 

 absorption of the yolk would seem therefore to be effected entirely by blood- 

 vessels. 



The yolk-sack persists long after hatching, and is gradually 

 absorbed. There is during the stages either just before hatching 

 or shortly subsequent to hatching (Cyprinus) a rich vascular 

 development in the mesoblast of the yolk-sack. The blood is 

 at first contained in lacunar spaces, but subsequently it becomes 

 confined to definite channels. As to its exact relations to the 

 vascular system of the embryo more observations seem to be 

 required. 



The following account is given by Rathke (No. 72*) and Lereboullet 

 (No. 71). At first a subintestinal vein (vide chapter on Circulation) falls into 

 the lacunae of the yolk-sack, and the blood from these is brought back direct 

 to the heart. At a later period, when the liver is developed, the subintes- 

 tinal vessel breaks up into capillaries in the liver, thence passes into the yolk- 

 sack, and from this to the heart. An artery arising from the aorta penetrates 

 the liver, and there breaks up into capillaries continuous with those of the 

 yolk-sack. This vessel is perhaps the equivalent of the artery which supplies 

 the yolk-sack in Elasmobranchii, but it seems possible that there is some 

 error in the above description. 



BIBLIOGRAPHY. 



(55) Al. Agassiz. " On the young Stages of some Osseous Fishes. I. Deve- 

 lopment of the Tail." Proceedings of the American Academy of Arts and Sciences, 

 Vol. xm. Presented Oct. n, 1877. 



B. III. 6 



