METABOLIC GRADIENTS OF VERTEBRATE EMBRYOS. 2"J 



as the chief abnormalities observed by him in the chick embryo, 

 (i) Duality and other anomalies of the heart. Dareste correctly 

 ascribed the double heart to an inhibition of the union of the 

 two heart tubes. (2) Blastoderms without embryos. In these 

 cases the blastoderm consists only of ectoderm and entoderm, 

 mesoderm being absent. (3) Anomalies of the area vasculosa, 

 absence of blood islands and blood vessels. (4) Anomalies of 

 the amnion and the allantois, due to inhibition at the time when 

 these membranes are forming. Dareste thought many other 

 abnormalities were caused by the suppression of the amnion, but 

 he was probably incorrect in this opinion. (5) Asymmetries of 

 various kinds, as of the eyes and two sides of the head. (6) Spina 

 bifida, correctly ascribed by Dareste to inhibition of the closure 

 of the neural folds. (7) Omphalocephaly, a condition in which, 

 due to suppression of the development of the head, the head is 

 reduced to a crumpled mass bent on the yolk (hence the name 

 omphalocephaly) and the heart is situated anterior to the head. 

 A typical case of omphalocephaly is illustrated in my Fig. 49. 

 Dareste notes that omphalocephaly is nearly always accompanied 

 by grave defects of the brain, eyes, and olfactory sacs. (8) 

 Various modifications of the brain and neural tube. (9) Modi- 

 fications of the eyes. (10) Celosomia or protrusion of the viscera 

 ventrally, due to failure of the lateral limiting sulci to close below, 

 (n) Failure of development of the limb buds and other abnor- 

 malities of the limbs. 



From this review of literature it seems evident that the 

 described abnormalities in the chick correspond with the expec- 

 tations from the susceptibility data in-so-far as the latter are 

 available. It has been shown in this paper that the parts of 

 the embryo most susceptible to toxic agents and hence expected 

 to be most amenable to modification are the anterior and pos- 

 terior ends of the embryonic axis, the special sense organs, 

 particularly the eyes, unions, crests of folds, bends, turns, etc., 

 and the limb buds. It will be seen from the long list of abnor- 

 malities just reviewed that these parts are indeed those which 

 most frequently exhibit abnormal development after subjection 

 to a variety of experimental conditions. The susceptibility of 

 certain other parts which seem frequently to exhibit abnormal 



