704 



Teratogenesis 



ble morphogenetic interactions. We know 

 little or nothing about the events which lead 

 to the initial cartilage abnormalities in the 

 grey lethal rat, which cause presumptive tail 

 tissue to degenerate in rumpless chicks, etc. 

 Most authors still accept Stockard's ('21) 

 four principles as explanations of causation. 

 These are, in his own words: 



Probably the most completely studied tera- 

 tological condition is that of micromelia (leg 

 shortening) in chickens. It may be profitable 

 to discuss this condition in some detail to 

 evaluate possible causal relations. 



Chondrodystrophy-like micromelias occur 

 frequently. They are found in many animals 

 besides chickens (man, cattle, dogs, rabbits. 



A B 



Fig. 246. A, An example of inherited chondrodystrophy in a 21 -day chick embryo (courtesy of Dr. 



W. F. Lamoreux, Cornell University, and the editors of the Journal of Heredity) . 



B, A markedly similar condition in an embryo from a hen which was fed a biotin deficient diet 



(courtesy of Dr. J. R. Couch and the editors of the Anatomical Record). 



1. "... Every type of developmental mon- 

 ster known in the literature may be produced 

 by one and the same experimental treatment. 



2. ". . . The same structural abnormality 

 may be induced in the embryos of various 

 species by a great number of different ex- 

 perimental treatments. 



3. ". . . In all cases the initial effect of 

 the experimental treatment is a lowering of 

 the developmental rate, and the resulting 

 deformity is always secondarily due to this 

 slow rate of development. 



4. ". . . The type of monster or deformity 

 is determined by the developmental period 

 during which the slowing in rate is experi- 

 enced." 



These principles preclude specificity of 

 action of the agents. They also preclude 

 specific tissue or rudiment requirements 

 which, in the presence of metabolic disturb- 

 ances, may result in an anomalous structure. 



salamanders). The condition is characterized 

 by a disproportionate shortening of the long 

 bones of the extremities, due largely to 

 growth retardation which is a consequence of 

 hypoplasia of epiphyseal cartilages. Fre- 

 quently the chondrocranium, especially the 

 jaw elements, is also distorted. As men- 

 tioned above, there are a number of inde- 

 pendent mutations in domestic fowl which 

 are responsible for this type of condition. 

 Similar chondrodystrophy-like syndromes oc- 

 cur sporadically (Landauer, '27) and have 

 been produced experimentally in chickens. 

 Byerly et al. ('35) reported the development 

 of short-legged embryos from eggs of chick- 

 ens on a deficiency diet which could be 

 mitigated by wheat germ. A similar anomaly 

 was reported by Lyons and Insko ('37); in 

 their case the dietary imbalance was re- 

 versed by manganese supplements. Roman- 

 off and Bauernfeind ('42) demonstrated that 



