268 PATTERNS AND PROBLEMS OF DEVELOPMENT 



Whatever the particular mechanism involved, it appears that in the 

 hybrid monsters, as in those resulting from injury to the germ cells, the 

 regions more susceptible to external inhibiting factors are, in general, 

 more inhibited by these physiological factors which interfere in some un- 

 known manner with the activities of life. The effect of the interference 

 is primarily greatest in the regions most intensely active, but in some cases 

 these regions apparently become more or less completely adjusted to the 

 physiological interference or "outgrow" it. One is tempted to suggest 

 that development of parts originating at high gradient-levels is more 

 "dilhcult" than that of other parts; that is, for such development the 

 mechanism must be in the best working order, and relatively slight inter- 

 ferences decrease its effectiveness more than that at lower levels, but the 

 "best working order" also involves, within limits, the possibility of more 

 rapid or more complete equilibration or adjustment to, or recovery from, 

 the lesser interferences. 



The enormous literature of descriptive teratology records numerous 

 monsters, chiefly in man and other mammals, apparently representing all 

 possible degrees of differential inhibition and perhaps in some cases sec- 

 ondary modifications. These have been variously classified and inter- 

 preted.^^ Little or nothing is known concerning the conditions determin- 

 ing these terata, but in the light of experimental teratogeny it is highly 

 probable that many physiological or pathological factors may give rise 

 to essentially similar modifications. 



A large and extremely interesting series of monsters occurring in certain 

 branches of an inbred strain of guinea pigs with relatively high frequency 

 has been described and its genetic basis discussed by Wright and Wagner 

 (1934; Wright, 1934). In this series a wide range of inhibited head forms 

 appears with resemblances to experimental differential inhibitions, but 

 most of them fall into two, rather than into a single, series. The chief 

 forms are designated and defined by Wright and Wagner as follows: 



Practically it turns out that a two-dimensional scheme includes nearly all of the 

 combinations described. The mandible is regularly more susceptible than the maxillary 

 process. This gives a basis for one series of grades. 



Brachygnathus: Mandible short but of normal width. 



Micrognathus: Mandible diminutive. Ventral approach of ears. 



P'ypoagnathus: Mandible absent. Ear ossicles united. 



Synzygo-agnathus: As above with more or less fusion of zygomatic arches. 



" See, e.g., St. Hilaire, 1832-37; Ahlfeld, 1882; Ballantyne, 1904; Schwalbe, 1906-13, par- 

 ticularly Teil III, Abt. i, Kap. 5 and 6, "Cyclopie, Otocephalie, Triocephalie, etc."; Abt. 2, 

 Kap. I, "DieMissbildungendes Auges," Kap. 2, "Missbildungen des Nervensystems," Kap. 5, 

 "Missbildungen des Gebisses," Kap. 6, "Die Missbildungen des Ohres." 



