248 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



of the fourth or fifth day, according to temperature, 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 oesophageal portion of the alimentary 

 canal immediately in front of the very elongate liver, and behind the 

 .benrt, with the venous sinus of which it appears to communicate by a 

 narroAv duct formed of the anterior portion of the yelk hyboblast, which 

 formerly covered the distended 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 sec- 

 tions. I 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 disapi^earance. 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 externally, 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 ap- 

 l)ears to be to build up the structures of the growing embryo; during 

 the second, not so much to build it uj) as to sustain it in vigorous health 

 until it can capture food to swallow and digest, so that it may no longer 

 be dependent upon the store of food inherited from its parent. 



The airpearance of the teeth. 



Minute conical teeth make their appearance on the lower jaws and 

 in the pharynx of the young shad about the second or third day after 

 hatching. Sections through the heads of embryos show that these 

 teeth are derived from the oral, hypoblastic lining of the mouth. There 

 are none on the upper jaw, there are four arranged symmetrically on 

 the lower jaw, or rather, Meckel's cartilage. In the throat, in the vicin- 

 ity of the fifth and last branchial arch, there are two rows of lower pha- 

 ryngeal teeth, the first of six, three on a side, the last of four, two on a 

 side. These teeth are of the same form and size as those on the jaws. 



The age at which it begins to talcefood. 



Although iDeristaltic contractions of the walls of the intestine of 

 young shad may be observed soon after hatching, I have never ob- 

 served food in the alimentary canal until ten or twelve days after the 

 young fish had left the Qgg. At about the beginning of the second 

 week considerable may be seen in living specimens. But the intes- 



