METABOLIC GRADIENTS OF VERTEBRATE EMBRYOS. 35 



In studying the metabolic gradients by the direct susceptibility 

 method, the observer watches the time of death of different levels 

 of the organism. It is evident that the completeness and accuracy 

 of his observations depend on two factors : first, the ease with 

 which the death point can be recognized, and second, the visi- 

 bility of various parts of the organisms. In regard to the first 

 matter, it may be said that the death point is usually recognized 

 by certain changes in the appearance of the part under observa- 

 tion ; it becomes white and opaque and loose and finally expands 

 into a shapeless mass of granules. In the case of vertebrate 

 embryos, owing to their delicate, almost diaphanous, structure, 

 these changes are not always detectable with certainty, especially 

 in very early stages, and repeated observation has often been 

 necessary in order to make certain of the course of disintegration. 

 As concerns the second matter, it should be perfectly obvious, 

 that the gradients can be determined only for those parts of the 

 organism which can be seen clearly ; in general these will be the 

 superficial parts. It is usually impossible to observe the death of 

 the entodermal structures ; such is the case in the present study. 

 The transparency of embryos, however, permits more extended 

 observations on internal structures than is usually possible with 

 adult organisms. 



II. DISINTEGRATION GRADIENTS OF FISH EMBRYOS. 



i. Material and Method. The disintegration gradients were 

 investigated in the embryos of three species of fish the killifish, 

 Fundulus heteroclitus, the cunner, Tautogolabrus (Ctenolabrus*) 

 adspersus, and the cod, Gadus morrhua. They were obtained at 

 Woods Hole, Mass., the first two species in June and July, and 

 the cod in December, 1919. Sexually mature Fundulus and 

 Tautogolabrus were obtained through the supply department and 

 were stripped " dry," that is into vessels containing no or very 

 little water. After fertilization had occurred water was added. 

 In this way a high percentage of developing eggs is obtained, as 

 previous investigators have found. The eggs of the cod were 

 obtained from the Bureau of Fisheries at Woods Hole and I am 



