158 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



erel is now from side, to side, tlirougli the eyes, tlie body seeming to be 

 concentrated towards the head. The breast fin hf, Fig. 14, is still a 

 semi circnlar lobe, with its base nearly horizontal, but it has been 

 advanced forwards somewhat twelve hours after hatching. It is merelj^ 

 a flatteued epiblastic i^ouch into which has grown a tract of mesoblas- 

 tic tissue. Twenty-one hours after hatching the breast fin has acquired 

 a vertical position, as shown in Fig. 15, while the coraco-scapular car- 

 tilage CSC is in its incii^ient stages of development in its base. Up to 

 this time the intestine has maintained its pilmitive character as a hor- 

 izontally flattened tube, which it will not begin to lose for twenty-four 

 hours more, but below the breast fin a swelling has api^eared in its lower 

 wall, Zr, which is the rudiment of the liver. 



Embryos on the fourth day after development have the appearance 

 shown in Fig. 16. The most marked advance which has made over 

 the stage, shown in Fig. 15, is that tlie intestine has acquired a cylin- 

 drical form, and is hollow or tubular, while it also has been bent upon 

 itself in its middle region. The liver is now a very prominent ventral 

 sack-like outgrowth from the lower side of the intestine at li\ just in 

 front of the bend in the alimentary canal. Its structure already shows 

 a lobular character, its walls bemg subdivided into the rudiments of 

 hepatic follicles. The epithelium over the rest of the inner surface of 

 the intestine becomes differentiated into follicles at a very early period, 

 which would indicate that the mucous membrane of the intestine had a 

 specific, probably digestive function, as soon as the follicular structures 

 were developed, which is accomplished about the time the young fish 

 commences to feed. Outside of the epithelial layer the annular and 

 longitudinal muscular layer of the intestine appears about the same 

 time, and peristaltic movements of the intestinal walls begin to be mani- 

 fested almost as soon as the intestine becomes tubular. Above the 

 liver, and a little way behind it, a diverticulum appears on the fourth 

 day, usually more or less obscured by pigment cells, which I regard as 

 the rudiment of the air-bladder ; by the seventh day it is much more 

 plainly developed, as shown in Fig. 17, ab. On the fourth day the 

 young fishes will begin to feed, as represented in Fig. 16, where the 

 black mass / represents the remains of food in the hind gut which the 

 animal had swallowed. From an examination of several specimens I 

 am not able, however, to state what this food was, as it was in a much 

 too disorganized condition to tell of what it originally consisted. The oil 

 sphere OS is now nearly absorbed as well as the yelk, which is entirely gone. 

 A portion of the anterior wall of the yelk sack, however, has remained as 

 a septum, partially shutting oif the pericardiac cavity from the body cav- 

 ity, in which the viscera are contained ; it stretches across between the 

 lower ends of the coraco-scaimlar cartilages esc. The circulation is now 

 fully established, there being an aorta and cardinal veins, cv^ which re- 

 turn below it to carry the blood back to the heart, a portion, however, 

 first passing through a vascular network over the viscera and represent- 



