THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 551 



components. For the most of the blood-corpuscles still continue to 

 lie in groups on the walls of the tubes, where they constitute the 

 previously described Hood-islands (fig. 114), which cause the red- 

 besprinkled appearance of the vascular area. The contractions of 

 the heart, by which the blood is set in motion, are at first slow and 

 then become more and more rapid. On the average, according to 

 PREYER, the strokes then amount to 130 150 per minute. How- 

 ever, the frequency of pulsations is largely dependent upon external 

 influences; it increases with the elevation of the temperature of 

 incubation and diminishes at every depression of it, as well as when 

 the egg is opened for study. At the time when the heart begins to 

 pulsate, no muscle-fibrillse have been demonstrated in the myocar- 

 dium ; from this results the interesting fact that purely proto- 

 plasmic, still undifferentiated cells are in a condition to make strong 

 rhythmical contractions. 



At the end of the third or fourth day the vitelline circulation 

 in the Chick is at its highest development ; it has undergone some 

 slight changes. We find instead of a single vascular network a 

 double one, an arterial and a venous. The arterial network, which 

 receives the blood from the vitelline arteries, lies deeper, nearer to the 

 yolk, while the venous spreads itself out above the former and is 

 adjacent to the visceral middle layer. The circulating blood is 

 distinguished by the abundance of its blood-corpuscles, the blood- 

 islands having entirely disappeared. 



The function of the vitelline circulation is twofold. First it serves 

 to provide the blood with oxygen, opportunity for acquiring which 

 is afforded by the whole vascular network being spread out 

 at the surface of the egg. Secondly it serves to bring nutritive 

 substances to the embryo. The yolk-elements below the entoblast 

 are disassociated, liquefied, and taken up into the blood-vessels, by 

 which they are carried to the embryo, where they serve as nutrition 

 for the rapidly dividing cells. Thus far the embryonic body 

 increases in size at the expense of the yolk-material in the yolk- 

 sac, which becomes liquefied and absorbed. 



The system of vitelline blood-vessels in Mammals agrees in general 

 with that of the Chick, and is distinguished from the latter only in 

 some unimportant points, which do not need to be discussed. How- 

 ever, this question certainly arises What signification has a 

 vitelline circulation in Mammals (fig. 134 ds) in which the egg is 

 furnished with only a small amount of yolk-material ? 



Two things are here to be kept in mind ; first, that the eggs of 



