312 THE SEGMENTATION OF THE OVUM. 



The Maturation, Fertilization, and Segmentation of the Ovum in White Mice. 



These animals are selected for the practical study of the earliest stages of 

 development for two reasons : first, because the processes have been more thor- 

 oughly studied in them than in any other mammals; and, second, because they 

 are easily kept and breed freely, so that abundant material may be secured with 

 comparatively little trouble. Those desiring further information are referred 

 to Sobotta's original memoir.* 



Heat occurs twenty-one days after littering, a fact which may be taken 

 advantage of to secure ova of the desired age. Coitus can take place only during 

 heat, for it is then only that the vagina is found open; at other times its epithe- 

 lium concresces to a solid mass. The spermatozoa do not penetrate into the 

 tube until some time after the coitus. After the discharge of the semen, the 

 contents of the large seminal vesicle are ejaculated into the vagina, completely 

 filling it and hardening into a white plug (bouchon vaginal), as in guinea-pigs. 

 From twenty to thirty hours later the plug softens and falls out. 



The Fallopian tubes are narrow, much contorted canals. The fimbriate 

 opening of the tube penetrates the connective tissue about the ovum so that the 

 fimbriae lie in the periovarial space. There is ciliated epithelium in the proximal 

 region of the tube only, none in the distal parts or in the uterus itself. During 

 heat the periovarial space is filled with an abundant clear fluid. This also dis- 

 tends the proximal part of the tube, forming, as it were, a special sac, with a dis- 

 tended epithelial lining. At the time of coitus ovulation has generally taken 

 place; the ovum, still surrounded by the cells of the corona radiata, is found in 

 the fluid of the distended proximal section of the tube. It is probable that the 

 ova are carried from the periovarial space not only by the currents created by the 

 cilia of the fimbriate opening, but also by a sort of pumping action of the tube 

 itself. For at the beginning of the period of heat we find that the periovarial 

 space contains much fluid, but later, when the ova are in the tube, this space is 

 empty and the tube contains fluid. The ovum of the mouse measures only 59 // 

 in diameter, and is, therefore, the smallest known mammalian ovum. It is 

 surrounded by a very thin zona pellucida measuring about 1.2 /i, and contains 

 only a few yolk-grains, a portion of which may be blackened by osmic acid. 

 These ova offer the further special peculiarity that they sometimes form two 

 and sometimes (from 80 to 90 per cent, of the cases) only a single polar globule. 

 When two are formed, the first appears while the cell is still in the ovary, the 

 second after it has been transferred to the Fallopian tube; when only one is 



*"Die Befruchtung und Furchung des Eies der Maus," " Archiv f. mikroskopische Anatomic," vol. 

 XLV, pp. 15-93, Pis. II-IV (1895). 



