W. B. Kirkliatn — Maturation of the Egg of the White Mouse. 71 



the height of the breeding season contained some eggs in which the 

 first polar body had been ah'eady extruded and in which the spindle 

 for the second polar mitosis was fully formed. A majority of the 

 same ovaries revealed ovarian eggs at the end of the spireme or 

 with the first polar spindle. The eggs observed in the Fallopian 

 tube fall into two main groups: those which had not been fertilized, 

 and which therefore retained the second polar spindle, — some being 

 accompanied by the first polar body, but more without it — and those 

 which had been fertilized. The latter show the entering sperma- 

 tozoon and the cleavage stages. 



Sreeding Season. — Most animals, including man, which live in an 

 artificial environment where there is an abundant supply of food the 

 year round, have lost the habit, so common among wild animals, of 

 being in heat only during limited periods of each year, and are 

 capable of coming into heat at any season. Among domesticated 

 animals, the white mouse is a good example of this characteristic, 

 and both Tafani ('89) and Sobotta ('95) have found that, if kept 

 warm during the cold months, it breeds more or less freely at all 

 times. Sobotta ('95), however, found that white mice breed most 

 actively from the beginning of March to the end of September, and 

 in most animals, wild as well as domesticated, the sexual season or 

 seasons occur during the warm luonths. Thus, Rubasclikin (:05) 

 has found guinea-pigs to be most active sexually during August, 

 September, and the beginning of October; Keibel ('99) states that 

 deer are in heat at the end of July and the beginning of August : 

 Rein ('83) finds that the period of heat for rabbits extends (at Strass- 

 burg) from the end of March to the middle of Jul}^, and Eimer, 

 Benecke, and Fries ('79) have determined that in bats the period of 

 heat is in the autumn. 



The period of gestation in the white mouse was put down by 

 Tafani ('89) as about 20 days, while Sobotta ('95) has determined 

 that it is just 21 days. 



During the active breeding season, adult female white mice, as 

 found by Sobotta ('95), are in heat a few hours after parturition, 

 and the same is true of the guinea-pig, according to Rubasclikin 



(:o5)- 



The ovary of the white mouse measures about 2""° in diameter, is 



more or less spherical in shape, and is chiefly composed of large and 



small follicles, a mature follicle measuring about .35"" in diameter, 



A comparison of this oi'gan in the rat and the cat shows that the 



ovaries of these animals are ovoid in shape, and measure 3x5""" 



and 4x8"'", respectively. 



