BULLETIN OF THE UNITED STATES FISH COMMISSION. 179 



OBSERVATIONS ON Till] ABSORPTION OF THE YELK, THE FOOD, 

 FEEDING, AND DEVELOPMENT OF E.TlliiSCYO FISHES, COITIPHISINtt 

 SOME INVESTIGATIONS CONDUCTED AT THE CENTKAE HATCH. 



ERY, AlMIOltV BIILD1IVG, WASHINGTON, D. C, IN 1888. 



By JOHN A. KYDEK. 



I. — OBSERVATIONS ON THE MODE OF ABSORPTION OF THE YELK OF 



THE EMBRYO SHAD. 



The manner in which the yelk of fish ova is absorbed or incorporated 

 into the body of the young fish, especially in those forms in which no 

 vessels traverse the yelk bag', was for a long time a puzzle to me. The 

 yelk in all cases diminishes in bulk progressively, not suddenly. This 

 fact indicated that the process of absorption probably occupied a con- 

 siderable time, and that if a careful watch was kept up it might possi- 

 bly be that the observer would be rewarded by the discovery of the 

 manner of its accomplishment. With this object in view, the writer 

 carefully observed young shad which had but recently left the egg, and 

 in which it was to be inferred that the blood was about to be developed. 

 My reason for choosing this stage of development was this: I knew, for 

 example, that in species in which there was a complex system of vessels 

 traversing the surface of the yelk sac, the substance of the latter 

 seemed to be absorbed by those same vessels in the form of corpuscles, 

 which, as in the case of Tylosurus, were unquestionably derived from 

 the store of protoplasmic matter embraced by the yelk bag. Aided as 

 I was by previous observations, which led me to undertake the investi- 

 gation, the result proved that I was right in my anticipations regarding 

 the manner in which the yelk of the young shad was absorbed, and that 

 the heart, practically the vascular system here, was one of the princi- 

 pal agents in the process, notwithstanding the fact that no true vitel- 

 line vessels are ever developed in this species. 



In order that the process may be made clear to the reader, I have 

 represented the anterior portion of an embryo shad in the accompanying 

 figure, in which the relation of the heart to the yelk is shown as dis- 

 tinctly as possible. The sketch represents the head end of the embryo 

 with the greater part of the yelk bag within the field of view, the whole 

 being treated as a transparent object enlarged about 35 times. In or- 

 der to understand the process of yelk absorption, to be hereafter de- 

 scribed, it will be necessary for us to know the relation of the yelk to 

 the rest of the embryo. The great mass of the yelk in the shad embryo 

 at the present stage of development is composed of coarse, irregular 

 masses of very clear protoplasmic matter, y. 



These irregular masses in turn seem to be separated from each other 

 by a material which occupies the fine interstices between them. It, 

 again, is a protoplasm optically different in character from that of the 



