DIFFERENTIAL DEVELOPMENTAL MODIFICATION. Ill 269 



Azygo-agnathus: Zygomatic arch absent or vestigial. No mandible. 



Premaxillary defect has no regular relation to this series, and the same is true of 

 defects of the nose, brain and eyes but premaxillary defects always seem to occur with 

 the latter group and these usually occur in a regular sequence of increasing susceptibil- 

 ity from anterior to posterior. This gives the basis for another series of grades. Most 

 of the terms are familiar in the literature of the subject. 



Brachyrhynchus: Premaxillary reduced or absent. Brain normal. 



Arrhinencephalus: As above except defect of olfactory lobes and more or less union 

 of cerebral hemispheres. Usually one nostril (monorhinus). 



Rhinocyclops: Forebrain vesicular. Eyes more or less united. Proboscis above the 

 single orbit. 



Cyclops arhinus: As above except for absence of proboscis. 



Anops: As above except for absence of eye. 



Aprosopus: Cerebellum and anterior parts of brain absent. 



Monoto-aprosopus: As above, ear vesicles united. 



According to the interpretation offered by Wright and Wagner, differ- 

 ent regions of the developing head attain their maximum susceptibiHty 

 at different stages, and the character of the individual modification de- 

 pends on the stage at which the inhibiting or depressing factor becomes 

 effective. That genetic factors are concerned in these modifications has 

 been demonstrated by Wright (1934). He suggests that they act by bring- 

 ing about, directly or indirectly, a general depression of vital activity at 

 a particular critical moment with permanent effects on the development 

 at the anterior end of the polar embryonic axis as the most active, and 

 hence the most susceptible, region at the time when the action takes 

 place. The depressing action at an earlier stage produces different modi- 

 fications from action at a later stage. The same is true for action of ex- 

 ternal inhibiting agents. But chance conditions also play a part in deter- 

 mining the individual forms; chance differences in implantation or in 

 blood supply or chance mutations are suggested by Wright and Wagner 

 as possible factors. "The randomness of occurrence within litters of each 

 size indicates that each monster is due to a highly localized chance event, 

 supplementing a genetic tendency common to all members of the group" 

 (Wright, 1934, p. 502). 



In an inbred strain of mice Little and Bagg (1924) obtained forms with 

 reduced lower jaw and frequent microphthalmia, apparently representing 

 somewhat less extreme degrees of differential inhibition than the guinea 

 pigs. Occurrence occasionally in mammals of a single individual or a 

 single litter showing characteristics of differential inhibition has been at- 

 tributed to various factors — for example, to maternal toxemia associated 

 with disease or other conditions — but practically nothing is known con- 

 cerning the particular factors involved in many cases. Results of experi- 



