28 CHARLES R. STOCKARD 



to the blood cells having wandered away from the mass since 

 they have largely degenerated in situ probably on account of 

 lack of aeration. The blood islands on the yolk-sac maintain 

 their red color for a much longer period of time, and they continue 

 to present a pattern closely identical with that seen in the same 

 individual during its earlier stages. 



6. Condition of the heart 'in old embryos without a circulation 



The conditions of the heart in some of these old embryos is 

 well shown in the series, figures 15 to 20. These figures are 

 from embryos of sixteen days old. The control specimens at 

 this time would as a rule have hatched. In the sketches the 

 peculiarly distended pericardium is strikingly shown. This 

 great distention of the pericardium seems to have exerted pres- 

 sure in such a way as to have straightened the anterior end of 

 the embryo and lifted it well away from the yolk surface. The me- 

 chanical pull caused by the separation of the head from the yolk 

 would seem to be largely responsible for the fact that the heart 

 becomes stretched into a very much attenuated tube or string. 



In the upper left hand figure 15, the heart is not so greatly 

 stretched and the pericardium in this case is not distended 

 so much as in the others. The upper right hand figure 16, 

 and the two central figures, 17 and 18, show the pericardium 

 distended to its utmost, and in these specimens the heart is 

 pulled out into a mere string. Pigment cells seem invariably 

 to wander along these string-like hearts, and they, therefore, 

 stand out in the embryos as a black cord just as is indicated in 

 the figures. The venous end of the heart which is connected 

 with the yolk-sac is seen at each pulsation to lift slightly the yolk 

 membrane in a cone-like projection from the surface of the 

 yolk. As these hearts pulsate in their feeble fashion, one thus 

 observes the yolk membrane as it is pulled up and down. 



The two lower figures, 19 and 20, show other somewhat dif- 

 ferent conditions of the heart. The hearts are small and do 

 not reach so far towards the ventral portion of the yolk. 



There is an almost limitless variety of peculiarly abnormal 

 hearts in these embryos and the six figures convey but a slight 

 idea of the many very strange conditions which are presented. 



