44 LIBBIE H. HYMAN. 



of the anterior end of the hindbrain ; this appears to be related to 

 the development of the auditory vesicles. In later stages evi- 

 dences of the appearance of a region of high susceptibility asso- 

 ciated with the formation of the cerebellum are present. 



6. The Gradient of the Fundulus Heart. This investigation 

 was in reality undertaken for the purpose of studying the gradient 

 of the heart. I was convinced from the facts known about the 

 heart that such a gradient must exist. My observations show that 

 such is the case ; however, the matter proved more difficult of 

 demonstration than was anticipated owing to the fact that the 

 outer surface of the heart does not readily undergo disintegra- 

 tion. It was only after repeated observation that it was dis- 

 covered that the disintegration processes occur only in the interior 

 of the heart, the surface outlines remaining intact. 



The gradient was studied in the Fundulus heart only, the hearts 

 of the other two species of fish being too small for the purpose. 

 The early stages of Fundulus proved unfavorable as the yolk 

 sac bulges up around the embryo and conceals the heart from 

 view. In later stages, after the heart beat is established, the dis- 

 integration gradient is invariably as follows. Disintegration 

 begins in the wall of the sinus venosus and progresses rapidly 

 along the heart tube to the arterial end of the heart. In this dis- 

 integration, the heart wall dissolves or melts away, leaving how- 

 ever, the external outlines intact ; this process sweeps rapidly 

 along the heart from the sinus to the bulbus arteriosus. This 

 gradient was observed in numerous cases in hearts in which there 

 was no visible differentiation along the cardiac tube. In later 

 stages after such differentiation has occurred the gradient is more 

 marked and steeper, that is, the disintegration passes more slowly 

 along the heart. After the interior has disintegrated the heart 

 tube expands but the outlines remain intact. 



The gradient in the heart is shown not only by the course of 

 the disintegration but also by the order in which the chambers 

 of the heart cease beating in toxic solutions. In younger hearts, 

 the order of cessation of beat is : bulbus and ventricle, auricle, 

 sinus venosus. The sinus continues to beat feebly after the 

 other parts of the heart have stopped contracting. The explana- 



