180 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



masses which it envelops. The relation of the clear masses to the mesh 

 work in which they are involved is shown in the figure. At the sur- 

 face of the yelk mass, and next to the homogeneous wall yhy of the yelk 

 sac proper, the clear masses become smaller, and in sections, if they 

 may be implicitly trusted, they sometimes present the appearance of 

 minute spheres or corpuscles. The immediate superficial envelope or 

 covering' of the yelk sac yliy is homogeneous, both in the living em- 

 bryo viewed as a transparent object, and also wlien examined in care- 

 fully prepared sections. This superficial layer is different, again, in 

 optical appearance from the clear masses and their matrix, already de- 

 scribed, so that the organization of the yelk is found to be quite complex. 

 This layer also covers the whole yelk, which is, therefore, truly a closed 



Explanation of figure. —Head and fore part of yelk sac of young shad, just batched, enlarged 35 

 times, y, clear yelk masses involved in a protoplasmic nieshwork j yhy, palish amber yelk hypoblast, 

 -which forms the innermost covering of the yelk; from its anterior portion blood-cells arc observed to 

 bo budding off into tho segmentation cavity c,- some are also noticed in the pericardial space p, and 

 in the heart;,- pm, the posterior pericardial membrane joined to tho heart and fused with the external 

 layers en a little way below j?m; i, intestine; nc, notochord ; t, rudiment of tongue, seen through the 

 transparent walls of the checks ; m, mouth ; I, margin of upper lip ; br, brain ; ««, ear ; bf, breast fin ; 

 1ns. spinal cord. 



sac or vesicle. In color this yelk covering is palish amber, quite differ- 

 ent from the clear body of the yelk, and at the anterior portion of the 

 sac, it is usually thicker than at any other point. In fact, just behind 

 the heart, which is inclosed in tin; space jt>, this superficial layer is often 

 heaped up in the form of a conical prominence, thus becoming several 

 times thicker at this point than at any other place. On the upper or 

 dorsal aspect of the yelk vesicle there is a longitudinal depression or 

 furrow, along which the superficial yelk envelope is depressed. In this 

 depression lies the cylindrical fore-gut i, which, for the most part, after- 

 wards becomes the oesophagus of the more advanced conditions of de- 

 velopment. There is no connection of any kind between the intestine 

 and the yelk sac at any time, such as has been described as connecting 



