38 ANATOMY 



are ripe between the thirty-sixth and fortieth days after birth, 

 about the time of the descent of the testes. The diploid number 

 of chromosomes in the sperm of the albino is 37. The haploid 

 is 19. 



(6) Oogenesis and fertilization. Ovulation is simultaneous in 

 both ovaries and there is a tendency for each ovary to discharge 

 the same number of ova. The number of young in the litter is 

 correlated with the number discharged during one cycle, but Long 

 and Evans ('22) found in the series of litters (averaging 6.4 

 young per litter) that the young represented about two thirds 

 of the ova discharged thus showing, from one course or another, 

 considerable loss. 



In a series of females isolated from the males Arai ('20) 

 determined the number of ova present in both ovaries between 

 the ages of one and 947 days. The results are given in table 

 10 and in chart 1. 



These data show a continuous decrease in the number of ova 

 with advancing age. The largest loss occurs before the end of 

 the suckling period. In this series the left ovary was on the 

 average 10 per cent heavier than the right, but the number of 

 ova on the two sides was nearly the same being three per cent 

 more in the right ovary. The appearance of the first corpora 

 lutea was most closely correlated with body length occurring 

 between 148 and 150 mm. equivalent in our reference tables 

 to a body weight of 80-85 grams and an age of 62-64 days. 



The ovum, after fixation with Zenker's solution containing 

 somewhat less than the usual proportion of acetic acid, meas- 

 ured 60-65 n in diameter with a nucleus about 25 M in diameter. 

 (Sobotta and Burckhard '10). The authors incorrectly as- 

 sume that the common Albino is a variety of Mus rattus. 



For the diameter of the living unsegmented egg Kirkham and 

 Burr ('13) give 79/z as a mean value. 



For the volume of the ovum see table 12. 



(c) On the early stages of development. We have the observa- 

 tions of Huber ('15 a). His description is as follows: 



In the albino rat the segmenting ova pass from the oviduct to the 

 uterine horn at the end of the fourth day after insemination, probably 



