[13] AN INDIRECT SOURCE OF THE FOOD OF FISHES. 767 



rhinus with their branchial sieves are particularly noteworthy for the 

 perfection of the apparatus of prehension, but we must not forget that 

 the gill-rakers of all fishes, whenever developed to any extent, probably 

 subserve a similar function. Lastly, the right-whales, with their closely 

 ranged plates of baleen suspended from the upper jaws, forming in 

 reality a huge strainer or filter for the large volumes of sea-water which 

 pass through the mouth, and from which the food of these marine giants 

 is so simply obtained, will enable us in a measure to comprehend the 

 importance of the minute life of the world, and its indirect but impor- 

 tant economical relation to man. 



THE POOD OF THE YOUNG SHAD. 



The periods of yeUi-ahsorption. — In a previous paper by the writer on 

 the retardation of the development of the shad it was stated that the 

 yelk-sack disappeared on the fourth to the fifth day after the young fish 

 had left the egg. Although this statement is in a broad sense true, I find 

 upon more accurate investigation that there is a small amount of yelk re- 

 tained in the yelk-sack for a much longer time. It appears in fact that 

 there are really two periods of absorption of the yelk which may be very 

 sharply distinguished from each other. The first extends from the time 

 of hatching to the end of the fourth or fifth day, according to tempera- 

 ture, during which time the most of the yelk is absorbed. The small 

 quantity which remains after this time is not visible externally, being 

 contained in a small fusiform sack, all that remains of the true yelk- 

 sack inclosed by the abdominal walls, and causes little or no visible 

 prominence on the under side of the young fish. Viewed as a living 

 transparent object from the side, we see it in the young fish lying below 

 the (esophageal portion of the alimentary canal immediately in front of 

 the very elongate liver, and behind the heart, with the venous sinus of 

 which it appears to communicate by a narrow duct formed of the an- 

 terior portion of the yelk hyboblast, which formerly covered the dis- 

 tended yelk-sack. The appearances presented by the living transparent 

 objects are fully confirmed by the evidence obtained from transverse 

 sections of embryos from ten to twelve days old. It appears that the 

 yelk-sack of the California salmon probably behaves in a somewhat 

 similar manner as indicated by transverse sections. 1 even find this 

 slight rudiment of the yelk-sack in shad embryos fourteen to sixteen 

 days old, but this seems to be about the period of its disappearance. 

 The second period of the absorption of the yelk therefore extends in the 

 shad over about twice that of the first, or about ten days. The first 

 period extends to the time when the yelk-sack is no longer visible ex- 

 ternally, the second from the time the remains of the yelk-sack become 

 inclosed in the abdomen until its final and complete absorption. The 

 function of the yelk-sack during the first period appears .to be to build 

 up the structures of the growing embryo ; during the second, not so 

 much to build it up as to sustain it in vigorous health until it can cap- 



