CHAPTER IV. 



STUDY OF THE SEGMENTATION OF THE OVUM AND OF THE 

 BLASTODERMIC VESICLE IN MAMMALS. 



In selecting material for general laboratory work on the early stages of mam- 

 mals, we are governed by practical considerations. The white mouse and the 

 rabbit are both easily kept in the laboratory and their breeding may be accurately 

 determined. Up to the present time the earliest phases of the development of the 

 mammalian embryo have been far more thoroughly studied in the white mouse than 

 in any other mammal. For the next following stages the same remark applies to 

 the rabbit. Hence these two forms have been chosen for the practical study. 



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 thoroughly 

 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 compara- 

 tively little trouble. Those desiring further information are referred to Sobotta's 

 and Kirkham's original memoirs.* 



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 epithelium 

 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 uterine 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 distends the 



* Sobotta, "Die Befruchtung und Furchung des Eies der Maus," Arch. /. mikrosk. Anal., vol. XLV, 15-93, 

 PI. II-IV (1895). 



Kirkham, "The Maturation of the Egg of the White Mouse." Trans. Connecticut Academy, xiu, 65-87, 

 PI. I-VIII. (Corrects several important errors of the preceding paper.) Also, Biol. Bull., (1910) XVIII, 245. 



160 



