46 LIBBIE H. HYMAN. 



side of the heart is more susceptible than the left. This greater 

 activity of the right side results in the bending of the heart to 

 the right. Depressing conditions would tend to prevent this 

 bending, so that the heart would remain as a straight tube. 

 I have also shown that in general the sinus end of the heart has 

 the higher rate of activity and that this decreases towards the 

 arterial end. Inhibiting factors might be expected to affect the 

 sinus end of the heart more than the arterial end, so that hearts 

 with small sinuses and enlarged arterial ends might be produced. 



Both of these expectations have been realized in certain 

 abnormal chicks produced during my study. These chicks have 

 already been referred to in the preceding paper. They arose 

 as a result of incubation for several days at 30 C., followed by 

 incubation at 39 C. for 24 hours. That development had 

 occurred at 30 is proved by the fact that all of these chicks 

 showed a degree of development corresponding to 40 to 72 

 hours of incubation. All of this lot of chicks were abnormal 

 and in many of them the heart was abnormal. As the other 

 abnormalities were considered in the preceding paper, only the 

 heart concerns us here. The abnormalities were of two kinds: 

 (i) the persistence of the straight form of heart tube at a stage 

 of development in which the heart would normally be con- 

 siderably curved; and (2) a reduced sinus region with an ab- 

 normally enlarged arterial end. The first type of abnormality 

 is illustrated in my Fig. 40. This embryo has thirteen somites 

 but the heart lacks the normal curvature. The arterial end is 

 also abnormally large. Fig. 41 is typical of the second kind of 

 abnormality. In this embryo of seventeen somites the sin- 

 auricular region is quite small as compared with the normal 

 but the anterior part of the heart is considerably enlarged. 

 This heart also does not bend to the right to the normal extent. 

 It should be stated that all of these hearts were living and beating 

 at the time when the embryos were fixed. 



It thus appears that the teratological development of the 

 heart corresponds with the expectations on the basis of the 

 susceptibility results. 



