METABOLIC GRADIENTS OF VERTEBRATE EMBRYOS. 2Q 



closure of folds, etc., result primarily from cell activity and not 

 from mechanical causes. 



e. Intense metabolism is a necessary condition for active 

 development. 



/. The chordate embryo in general develops through the 

 formation of two centers of activity, one of which is situated 

 at the anterior end of the embryonic axis and develops into 

 the head, and the other of which, more posteriorly located, is 

 a growing point which progressively shifts posteriorly, depositing 

 the trunk of the embryo anterior to it. This posterior center 

 of activity is the dorsal lip of the blastopore in amphibian 

 embryos, that portion of the teleost embryo which is situated 

 in the germ ring, and the differentiating region of the primitive 

 streak in the chick. 



g. Whether or not the chordate embryo develops with a 

 primitive streak appears to depend on the relative time of 

 appearance of the posterior center of activity with respect to 

 the time of gastrulation. If the posterior center is established 

 at the time of gastrulation, development occurs without the 

 formation of a primitive streak; if later, a primitive streak 

 arises. 



3. A review of the literature on induced teratological develop- 

 ment in the chick is given and an explanation of such develop- 

 ment is suggested on the basis of the present results. It is 

 pointed out that those portions of the embryo shown in this 

 paper to be most susceptible to lethal solutions also most fre- 

 quently exhibit abnormal development under the action of 

 agents of a general depressing nature. Such agents or conditions 

 in proper concentration and duration affect the course of develop- 

 ment in rough proportion to metabolic rate, those parts of the 

 embryo requiring the highest metabolic rate for their normal 

 development or having the highest rate at the time of application 

 of the condition being more affected than parts developing at 

 lower metabolic levels. 



BIBLIOGRAPHY. 

 Adelmann, H. B. 



'22 The Significance of the Prechordal Plate: an Interpretative Study. Amer. 

 Jour. Anat., Vol. 31, pp. 55-101. 



