ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 459 



reticulum, which according to the nature of its development is formed 

 gradually, differentiates into the cortex and the medulla in develop- 

 mental stages 65-75 mm. in length, and is fully formed in embryos 

 180 mm. in length. 



3. Third, there is the epoch of the formation of red blood-cells and 

 the development of granular cells. An occasional red blood-cell is found 

 in the thymic lobules shortly after lymphocytes are found in them. 

 They are, however, first present in appreciably large numbers in stages 

 of about 55 mm. in length, and are most numerous in the thymus of 

 full-term embryos. In the interlobular septa of the thymus the greatest 

 number occurs in stages of about 125 mm. in length, while only a few 

 are found in embryos of ISO mm. in length to full term. Eosinophile 

 cells were first found in thymic lobules of a 42 mm. embryo, but occur 

 first in appreciably large numbers in embryos of about 180 mm. in 

 length, and are most numerous in the parenchyma of the thymus of 

 full-term embryos. In the interlobular septa they are seldom found 

 in embryos from 65-83 mm. in length ; they occur first in appreciably 

 large numbers in the septa of embryos of about 125 mm. in length, and 

 are most numerous in embryos 165-185 mm. long, but are still present 

 in the septa in full-term embryos. 



Experiments on White Mice.* — Frances B. Sumner gives an 

 account of his studies (1906-1911) of environmental influence, heredity, 

 correlation and growth in the white mouse. About 2300 animals were 

 measured. The parent generation of mice was reared, usually from 

 birth, in two rooms differing widely in temperature and humidity. In 

 some cases a second generation was reared in the two rooms. Sumner 

 found certain differences between the mean measurements of lots which 

 were reared in the cold-room and in the warm-room. As regard the tail 

 and the foot these differences were considerable in amount and of 

 absolutely certain statistical significance. They were always in favour 

 of the warm-room animals. The cold-room animals were chiefly 

 modified. A modification in ear length, at first apparent, turned out 

 to be slight in amount and of inconstant occurrence. 



Comparison of seventy-four warm-room and fifty-three cold-room 

 male mice, killed before the withdrawal of the temperature differences, 

 showed that the latter had an appreciably greater quantity of hair. 

 Comparison of about fifty mature females, killed nearly six months after 

 the discontinuance of the temperature differences, showed more hair in 

 the warm-room lot. Temperature did not appear to have any constant 

 effect upon body weight. 



In the offspring of two sets of modified mice, born and reared in a 

 common room, those of warm-room parentage had, on the average, a 

 greater weight and greater length of tail, foot, and ear than those of 

 cold-room parentage, when animals of the same body length were 

 compared. Four lots of about two hundred each were measured. In 

 one of the four lots, born nearly five months after the discontinuance 

 of the temperature differences, determination of hair weight showed a 

 greater quantity in the warm-room series. While the offspring of cold- 



* Journ. Exper. Zooh, xviii. (1915) pp. 325-432 (17 figs.). 



