BULLETIN OF THE UNITED STATES FISH COMMISSION. 157 



In Fig-. 14 a large siuus, ss, is shown just over tlie brain, wliicli is 

 roofed over by the dermal and deeper layers of Ihe epiblast; the cavity 

 so formed is filled with flnid and persists for at least a week, as shown 

 in Fig. 17. Below the hinder part of the head and at its sides, in 

 Fig-. 14, the branchial farrows hr are visible ; as these become decider 

 they are finally broken throngh into the wide branchial chamber of the 

 fore-gut, as the gill-clefts. This takes j)lace about tAventy-four hours 

 after hatching, and is on the eve of accomplishmeut in Fig. 15, in 

 which the point where the mouth will appear is indicated at m on the 

 lower side of the head; just behind the point where the mouth will soon 

 break through, the rudiment of the lower jaw has made its appear- 

 ance as Meckel's cartilage, m?:. Above and behind it the cartilaginous 

 rudiments of the branchial skeleton or framework of the gills have been 

 in part developed. It will also be noticed that the head is higher and 

 that the brain is bent downwards more than in Fig. 14, in conse- 

 quence of the decrease in size of the yelk sack. The mandibular arch, 

 the forepart of which is indicated at mTc, does not yet reach forwards 

 on a line with the end of the snout. While the hyoid arch, or that im- 

 mediately following the mandibular, is still more or less obscured by 

 the latter, and covered up by it externally, behind which the branchial 

 arches or rudimentary gills are more crowded together from before 

 backwards than in the stage shown in Fig. 14. The trabecul?e cranii 

 have, however, been developed, but are still rudimentary, being pres- 

 ent only as a pair of symmetrically disposed longitudinal rods beneath 

 the brain. The eye, which was not heretofore completely pigmented, 

 is now quite black and opaque, the choroid layer being- developed. The 

 ear and nasal pits have undergone farther development ; the latter are 

 present as distinct cups in front of the eyes. The heart has undergone 

 considerable advance, in that its jiosterior end is now directed upwards, 

 while the ventricle and bulbus aort?e are more fully developed and ac- 

 tively pulsating, although the blood system is stUl imperfect, no trae 

 aortic or venous channel having- as yet been developed ; however, there is^ 

 already a partial branchial and cephalic circulation, but the blood chan- 

 nels are not yet supplied with a sufficient number of blood corpuscles 

 to mark their courses distinctly by the color. The cavity in which the 

 heart noAV lies is bounded in front by the branchial structures, below by 

 what was formerly the anterior portion of the epiblastic sack covering; 

 the yelk, behind hj the hj-poblastic walls of the yelk sack, above by the 

 intestine and body walls. In this and earlier stages of develoi)ment j 

 believe that I have seen blood corpuscles swimming freely in the cavity 

 surroimding the heart, which would indicate that they had been derived 

 by budding directly from the hyx)oblast, Avhich is beyond the shadow of 

 a doubt the way in which they are developed in the embryo silver-gar, 

 or bill-fish {BeJone). The yelk sack is shown much reduced and more 

 strongly pigmented in Fig. 15, while the oil sphere has undergone- 

 considerable reduction in size. The widest portion of the young mack 



