Species crosses in Rats. j^Oy 



make them breed at all. Oiii- first litters in P'ranre were obtained in 

 a brick vaulted room in the ruins of a castle. In Hussum, Holland, we 

 constructed a few little lOoms of four feel in all directions, made of 

 asbestos-cement jilates, and later we found that some paiis would breed 

 in cages four foot deep and sixteen inches high and wide. In Buiten- 

 zorg, .lava, we bred our rats in similar cages, and in masonry tanks, 

 of approximately ^3 cubic feet content. Later we bred them in tin 

 boxes made on the plan of the herbarium boxes of th(; Botanical gardens, 

 and here in Berkeley we obtain fairly regular results in using wooden 

 cages of 8 X 12 ^ 24 inches, with nestcompartments. Gradually, by a 

 process of unconscious selection, through the fact that only those pairs 

 would have sufficient offspring which did breed in cages, our colony 

 was brought to breed in cages with sufficient regularity. The rats are 

 very susceptible to pneumococcus infections, although they are absolutely 

 immune to paratyphoid, which is such a nuisance in colonies of domestic 

 rats. They require food rich in protein, and apparently thrive best 

 when given fruit, green food and milk or similar things rich in vitamins. 

 The females very often refuse to breed when given too long a rest 

 after the birth of a litter. This is a common condition in rabbits, which 

 is called ''going stale" by the rabbitfanciers. 



The numbers bred by us are small, especially if compared to the 

 thousands of animals bred by the endowed institutions working with 

 domestic rats. We did what we could with our limited private means. 

 We now turn to those experiments showing the inheritance of factors 

 C, D, E, F, G, H, I and J. 



Yellow and Silver. 



C and I) are the genes which are lacking in yellow and silver, 

 cd rats are very light coloured. The parts of the hair, which are yellow^ 

 in agouti animals seem to have the same colour in these cd rats, 

 whereas the blackpigmented parts of the hair are pigmentless. All 

 hairs in these yellow rats have white tips, and the enormous vibrissae 

 are white throughout their length. Our first yellow and silver rats 

 were l)red in Bussum, Holland. We continued work with them in 

 Buitenzorg, Java, and in Berkeley, where the strain finally died out. 



In these yellow rats, presence or absence of factor B makes a 

 difference between whitebellied and darkbellied animals. 



Four litters of young were produced from the original whitebellied 

 yellow animals. They contained 19 whitebellied to 7 darkbellied yellows. 



