32 LABORATORY MOUSE 



This retinal defect (see Fig. 36) is characterized by com- 

 plete absence of rod and external molecular layers and a 

 great reduction of the cell number in the external nuclear 

 layer (86, 90). The condition is readily detected by examin- 

 ing histological sections of the retina. The iris of the rod- 

 less eye contracts (88, 89) upon exposure to light and the 

 blindness of rodless mice may be detected only by precise 

 and carefully conducted animal-behavior tests (87). Rod- 

 less eyes secrete no visual purple (90) and produce no elec- 

 tric action current responses (99) to stimulation by light. 

 The recessive gene producing rodless retina is linked (98) 

 with that producing silver. 



Dwarf (dtv) 



In the case of dwarf mice, mature individuals are only about one-fourth 

 the weight of their normal brothers and sisters, scarcely bigger, in fact, 

 than the ordinary mouse 16 or 17 days old. — Snell, 1929. 



These creatures (see Fig. 33) are probably pituitary- 

 deficient dwarfs. 1 The character is determined by a reces- 

 sive gene (160). 



Hyperglycemia (hy) 



The findings obtained with the second generation confirmed the con- 

 clusion that hyperglycemia, like albinism, is a recessive character. — 

 Cammidge and Howard, 1926. 



It has been definitely established (13) that a certain strain 

 of mice bear a gene, which in the homozygous state raises 

 the fasting blood-sugar proportions. Whereas normal mice 

 have a blood-sugar content of 74-84 mg. per 100 cc. of blood, 

 the mutant strain bears from 113-124 mg. per 100 cc. The 

 factor concerned is not linked with albinism but further 

 genetic analysis is wanting. 



Other characters. Numerous other mouse variations are 

 known to breeders of mice. Some of these characters are 



1 A paper by P. E Smith and E. C. MacDowell in the Anatomical Record, vol. 

 46, p. 249, published too late for inclusion in the bibliography, confirms that the 

 dwarfed condition is due to anterior pituitary deficiency and may be corrected by 

 injection of rat pituitary. 



