50 PHYSIOLOGICAL GENETICS 



Wright's analysis of the facts led him to the following conclu- 

 sions: In accordance with the experiments of Stockard, etc., 

 which were reported above, it seems that the monstrosity is 

 produced by an inhibitory action upon early stages of develop- 

 ment prior to the visible formation of the organ rudiments. 

 (This is, of course, found to be the case in all experiments on 

 determination.) But whereas the experimental treatment pro- 

 duces more diffuse and general effects, the genes in question have 

 a specialized effect of the type described as otocephaly. This 

 means, according to Wright, that the gene effects are relatively 

 direct and precise in their moments of action. In detail, Wright 

 favors Child's point of view : Any agent that inhibits the metabolic 

 processes of the early embryo before the primary simple gradient 

 pattern has become complicated affects especially the most 

 active region with consequences mostly upon the anterior end 

 of the medullary plate. 



Other explanations of the embryological facts are, however, 

 possible. Lehmann (1936) concludes from his experiments that 

 the primary cause for the formation of these monsters is probably 

 a defect in the head organizer. This sounds, indeed, more 

 reasonable than the idea of gradients, which has not much of a 

 factual basis. 



Reference has been made to the work of Little and Bagg 

 in which it was shown that abnormalities in mice caused by a 

 recessive gene and appearing phenotypically rather diversified as 

 clubfoot, Polydactyly, anophthalmia, etc., were the result of 

 blood extravasates formed in early embryonic life. A simple 

 embryological process then produced all those complex features. 

 A detailed analysis of all the defects of skull, mandibles, brain, 

 etc., combined in the picture of otocephaly showed that they all 

 may be traced to a small number of centers of abnormality: 

 ventral mandibular arch, olfactory placodes, cerebral vesicles, 

 median optic rudiment, and some others. "These in turn may 

 be related by the hypothesis that the basic factor in this class of 

 abnormalities is inhibition of the anterior medullary plate and 

 associated ectodermal placodes. ..." It may be that a 

 still earlier and more generalized stage is the critical one (see 

 preceding paragraph). The important point remains that an 

 early inhibition at a definite point of embryonic development is 

 responsible for the whole chain of later events. All the types of 



