W. B. Kirkhatn — Maluration of the Egg of the White Mouse. 79 



When the egg of the mouse has extruded its first polar body and 

 formed the second polar spindle, it normally never develops further 

 unless it is fertilized, but degenerates either within the ovary or in 

 the Fallo})ian tube, as stated by Sobotta ('95). According to Ru- 

 baschkin (:05), this is also the case in the guinea-pig's Qgg. These 

 investigators further state that in the animals studied by them a 

 considerable number of eggs reaching this stage fail to be discharged, 

 owing to their position in the interior of the ovary. This condition 

 has been also frequently observed in the mouse, by the present 

 writer. 



Ovulation. — It is an exceedingly rare thing to obtain a mamma- 

 lian egg just leaving the ovary, but such have been figured by 

 Barry ('39) for the rabbit ; by Sobotta ('95) for the mouse ; and by 

 van der Stricht (:0l) for V. noctnla. Little is known regarding the 

 factors concerned in ovulation, but the accumulation of fluid within 

 the follicle probably plays an inportant part and, judging from the 

 observations of lieape (:05) on rabbits, the presence of an abundant 

 blood supply to the ovary is essential. In the mouse, during the 

 active breeding period, the eggs leave both ovaries within an hour 

 or two after parturition, independent of copulation ; while* in the 

 rabbit, Barry ('39) and Heape (105) agree that ovulation occurs only 

 after coitus, and then after an interval of from 9 to 10 hours. 

 According to Reichert ('61), the guinea-pig, like the rabbit, ovulates 

 only after copulation, and the interval in this case is stated by Ru- 

 baschkin (:05) to be about 17 hours. 



Fertilization. — According to Sobotta ('95) there is but a single 

 copulation in the mouse. Normally only a single spermatozoon 

 enters an egg. The tail of the spermatozoon usually enters the egg 

 at least in part, and may be entirely carried in (PI. VII, fig. 14), 

 as observed by van der Stricht (:04) in the egg of V. noctula, and 

 by Rubaschkin (:05) in that of the guinea-pig. 



Second Polar Body (Pis. VI-VII, figs. 13-15 ; Text-fig. 3).— 

 In the mouse, the second polar body is formed only by those eggs 

 which are fertilized, as found by Sobotta ('95), and it appears very 

 soon after the entrance of the spermatozoon. A similar condition 

 has been observed in the guinea-pig by Rubaschkin (:05), except 

 that in this animal the second polar body is not extruded until the 

 sperm nucleus has penetrated deep into the egg. In Plate VII, fig, 

 14, is shown an egg in which the second polar body has been so 

 recently extruded that the 12 univalent chromosomes are still visible. 

 After a very short time these would have been collected into a moi'e 



