THE METABOLIC GRADIENTS OF VERTEBRATE 

 EMBRYOS. IV. THE HEART. 



LIBBIE H. HYMAN, 

 HULL ZOOLOGICAL LABORATORY, UNIVERSITY OF CHICAGO. 



The study of the disintegration gradients of the heart was 

 part of the work on the gradients of the chick embryo which 

 has been presented in the paper which precedes this one in 

 the same number of this journal. The investigation deals with 

 the heart of the chick embryo only. 



The methods employed were the same as recounted in the 

 preceding paper of the series. The chick embryos were mounted 

 ventral side up and the blastoderm which passes beneath the 

 heart generally removed with a needle after the disintegrating 

 agent had been applied and had begun to take effect. Unless 

 this blastoderm is removed the heart cannot be seen very dis- 

 tinctly. The agents used were the same as in the preceding 

 study, namely, potassium cyanide, ammonium hydroxide, and 

 sodium hydroxide, all made up in rather strong concentration in 

 isotonic sodium chloride solution. 



It has proved necessary to spend a great deal of time and 

 material in studying the death gradients of the chick heart. 

 The reason for this is that the heart commonly dies without 

 showing any very clear or obvious death changes. This seems 

 to be due to some peculiarity of texture or consistency in the 

 heart even in very early stages. Since the observer can naturally 

 determine the time of death of a structure only when death 

 changes of a visible nature occur, susceptibility conditions in the 

 heart have been difficult to study. The clearest death changes 

 occur in sodium hydroxide solutions but the same susceptibility 

 differences have also been noted when ammonia or potassium 

 cyanide solutions are employed. 



Immediately after the establishment of the medullary plate 

 the head of the chick embryo is delimitated from the blastoderm 

 by the formation of the head fold. This fold composed of 



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