BULLETIN OF THE UNITED STATES FISH COMMISSION. 291 



at v', v", iu Fig. 12, ninety-four and a lialf hours after development had 

 begun. The bU)od corpuscles or disks at once begin to be formed, but 

 they do not appear to be uniformly oval at first, but very soon acquire 

 the red color characteristic of this tissue. The corpuscles have a tend- 

 ency to adhere together in clusters or clumps, and circulate in this way 

 in masses through the blood channels of the embryo, as shown in Fig. 

 11 at V. The blood channels soon develop' communicating branches, 

 and these are formed in a very interesting way, as represented iu Fig. 

 13. Narrow blind prolongations of the hollow vessels are formed at. 

 their sides and at each pulsation of the heart these are lengthened; 

 several of them are shown at c, e, c, Fig. 13. Frequently two such blind 

 prolongations meet and join, so that a communication is established be- 

 tween the larger channels. In this way the vascular network is devel- 

 oped over the yelk, as shown in Figs. 14 and 16. In Fig. 14, one hun- 

 dred and sixteen hours after impregnation, the vitelline vascular system 

 is moderately comi)lex ; there are three vascular channels, the right one, 

 v', the left one, v", and the median one, t\ all of which join and pour 

 their contents into the venous end of the heart at a. It may also be 

 observed that where the vessels cross the semi-diameter of the egg at 

 the side and where their cavities are seen in optic section, the epiblast 

 is lifted up to give them passage. Between the vessels at this stage 

 it was possible to observe in optic section here and there at the sides 

 of the vitellus the space between the epiblast and hypoblast, which 

 we have regarded as the remains of the i^ersistent segmentation cavity. 

 The heart space p, which in this as in other cases appears to be de- 

 rived from the segmentation cavity, becomes progressively more and 

 more spacious in the successive stages represented in Figs. 11, 12, 14, 15, 

 until it attains a most extraordinary development in Fig. 16, one hundred 

 and sixty-five and a half hours after impregnation. In Fig. 1 1 the heart 

 is tubular and not differentiated into regions; in Fig. 12 the ventricle 

 and venous sinus are beginning to be marked off from each other; in 

 Figs. 14 and 15 the bulbus aortie may be for the first time distinguished, 

 but in all of these phases the whole organ is dragged forward in the 

 median line far beyond the front of the head. In Fig. 16 the venous 

 end of the heart begins to be inclined downward, but is at the same 

 time very remarkably elongated; the bulbus aorttie ha is almost tubular 

 and the ventricle ve, almost globular, is held iu position to the fioor of 

 the heart cavity by a muscular or fibrous band, s. Below the ventricle 

 the greatly elongated tubular veuous sinus appears to be fastened by 

 diverging muscular bands to the lower part of the enormous heart space^. 

 The point of attach ment of the venous end of the heart in the lower por- 

 tion of the heart space is the scene of the very remarkable mode of gen- 

 esis of blood corpuscles of this species. Where the vessels v, v', v", in 

 Fig. 14, converge, it is already apparent that an active metamorphosis 

 of the yelk substance into blood disks is in progress. The first sign of 

 this has, however, already made its appearance in Fig. 11, where the 



