182 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



ill which they are found, on account of the persistent pulsation of the 

 heart, I found it was impossible to ascertain whether they manifested 

 any amoeboid movements or changes of form such as may be ob- 

 served in the colorless blood cell. In the figure I have exaggerated 

 the number visible at one time at this stage in order to show more clearly 

 the steps of the process. In the embryo shad they are formed sparingly 

 at first, but at a later period of development they become more plenti- 

 ful. In TyJosurus they are, however, formed in such myriads at about 

 this same stage that it would be quite impossible to count them, the 

 serous fluid surrounding the heart being charged with vast numbers, 

 which are in this instance, however, already reddish in color, which is 

 not the case with the shad, where the red coloring matter of the blood 

 appears to be developed at a later stage. It is a singular general truth 

 that, in those species in which the pigmentation of the body takes place 

 early or while the embryo is still within the eg ( ^, the blood cells become 

 reddened much earlier than in those in which the pigmentation is delayed 

 or retarded. In fact, it also appears to be generally true that the first 

 lines of pigment cells are developed along the courses of the great blood 

 vessels and in the neighborhood of specialized sensory organs. 



This, however, is leading us away from the subject in hand. As the 

 yelk vesicle or sac diminishes in bulk it tends to become pointed ante- 

 riorly. The external layer of the yelk yhy, which, as we saw, affords 

 the material for new supplies of blood cells, becomes thickened ante- 

 riorly, and sometimes even appears to be extended into a conical point 

 directed towards the venous and open end of the heart. This would 

 indicate that the yelk was being consumed from its anterior extremity. 

 The anterior conical end of the yelk vesicle sometimes. presents a granu- 

 lar, or rather corpuscular, appearance after two-thirds of the whole has 

 been absorbed. This condition is in keeping with what we observed at 

 an earlier stage, where, as in the figure, we saw the outer layer of the 

 yelk gradually break up into corpuscles or spherules, which were taken 

 up by the heart, although as yet there was no evidence of a complete 

 circulation. Together with this diminution in the volume of the yelk 

 vesicle, the membrane pm is drawn back, at its outer attached border, 

 becoming more funnel shaped; into this infundibuliform backward pro- 

 longation of the posterior pericardial membrane the conical anterior 

 extremity of the yelk mass extends. Gradually the bulk of the yelk 

 diminishes still more until it remains as a fusiform mass which is no 

 longer prominent on the ventral side of the body pf the young fish. 

 Meanwhile the liver of the young fish has been more developed and the 

 portal vein makes its way over the dorsal aspect of the yelk towards 

 the venous end of tin- heart. It appears probable thai what now re- 

 mains of the yelk may be taken up in part by the portal vein, but of 

 this I am not well assured, further than to state that the portal vessels 

 or channels appear in part to traverse what was formerly the segmenta- 

 tion cavil \ c. 



