ORIGIN OF VASCULAR TISSUES 97 



displayed waves of contraction which started in the end nearer 

 the heart and proceeded ventrally to the opposite end. Evi- 

 dently this lacuna was provided with some sort of myocardium. 

 It had developed a dorsally directed flexure, as seen in figure 69; 

 the yolk endothelium is not shown in this sketch. At this time 

 the churning of the heart could be seen to loosen the closely 

 packed cells in the intennediate cell mass which had begun to 

 acquire a faint orange color. On the fifteenth day a careful 

 examination of the posterior region of this embryo revealed the 

 fact that certain erythrocytes were being dislocated by the move- 

 ment of the heart, and some could be followed to a large lacuna 

 on the yolk, although there was no complete circulation of the 

 blood. Such cells would oscillate slightly from the motion of 

 body-fluids caused by heart-pulsation. Blood lacunae on the 

 yolk may sometimes be of this sort of formation. The formation 

 of the endothelial tubes and cavities on the yolk was watched in 

 the living condition. As has already been seen, at the age of 

 ten days this activated lacuna, accessory heart, or whatever it 

 may have been, was pulsating feebly. On the nineteenth day 

 (fig. 70) the lacuna or 'accessory heart' was pulsating sev^enty 

 times per minute, while the real heart, seemingly normal in 

 shape, was pulsating one hundred times per minute; the latter, 

 was in no way connected with any yolk-sac vessel. Likewise no 

 endothelial tissue had ever approached, or formed near the ac- 

 cessory heart. The yolk-circulation was established through the 

 normal heart on the twenty-fifth day. At that time the rate of 

 heart-pulsation had greatly increased, especially in the accessory 

 heart. Rather suddenly on the twenty-fifth day the rate of 

 beat in the accessory heart increased to 136 times per minute 

 while that of the normal heart was 126 times per minute. The 

 accessory heart increased its rate somewhat, and then began to 

 decline considerably on the twenty-sixth day, when its rate be- 

 came slower than that of the normal heart. At this time the 

 embryo was killed. The flexure of the accessory heart had 

 greatly increased. It is difficult to explain the extraordinary 

 manner in which the rate of beat of the accessory heart was 

 changed. It is conceivable that we here have to do with the 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 21, NO. 1 



