THE METABOLIC GRADIENTS OF VERTEBRATE EMBRYOS. III. THE CHICK

Abstract

1. Differences in the time of death of different regions of chick embryos dying in toxic solutions are described. Chick embryos of various stages from the primitive streak through the third day of development were studied. In primitive streak and head process stages, there is a simple death gradient from the anterior to the posterior end of the embryonic axis. From the medullary plate stage on there are two regions of high susceptibility—the anterior end of the axis and the growing point of the primitive streak; from these two regions death progresses towards the middle of the embryo. Other structures showing high susceptibility at certain times are: the special sense organs, the limb buds, and all places where unions, folds, bendings, etc., are about to occur. A more detailed summary is given on p. 11.2. Application of the results to normal development leads to the following suggestions, susceptibility or time of death being regarded as roughly proportional to metabolic rate:a. There is a gradation in rate of metabolic activity along the primitive streak in an antero-posterior direction. This probably determines the cephalo-caudal progress of development.b. The head process is probably merely a further development of the anterior end of the primitive streak.c. The primitive streak (excluding the head process) forms all of the embryo from the first somite posteriorly.d. Such developmental processes as the union of paired primordia, formation of bends and flexures, occurrence and closure of folds, etc., result primarily from cell activity and not from mechanical causes.e. Intense metabolism is a necessary condition for active development.f. 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 development in the chick is given and an explanation of such development 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 frequently 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 development 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.