166 STUDY OF THE SEGMENTATION OF THE OVUM. 



protoplasm, and the line of separation of the two cells passes through the equator 

 of the spindle (Fig. 123, A). The accumulated granules of chromatin then reconsti- 

 tute the resting membranate nucleus (Fig. 123, B). In brief, the segmentation of 

 the ovum is a typical indirect or mitotic cell-division. In the mouse the first 

 cleavage is completed about twenty-six hours after the coitus. The second cleav- 

 age is not completed until twenty-four hours later. When first formed, the two 

 segmentation spheres are oval and entirely separated from one another, but subse- 

 quently they flatten against one another and become appressed, a phenomenon of 

 which we have no explanation. In most mammals which have been studied there 

 is more or less space between the segmenting ovum and the zona (see Fig. 6), but 

 in the mouse this space is reduced to a minimum and the zona is often stretched 

 into irregular forms during the changes of the ovum. 



Method of Obtaining Blastodermic Vesicles from the Rabbit. 



The does should be allowed to become pregnant and be isolated until they 

 have littered; the date of littering should be noted, and thirty days thereafter the 

 buck be admitted and the exact time of the covering recorded. At the proper 

 number of days thereafter the animal should be killed and the uterus removed at 

 once. It may be opened with two pairs of forceps used to split the outer muscular 

 walls of the organ, beginning the operation at the lower end of the uterus. With a 

 little care this can be done without rupturing the mucous membrane, which is to 

 be afterward also opened in a similar manner with the forceps and the blastoder- 

 mic vesicles exposed. They are small bodies of rounded form and with a brilliant 

 pearly luster, and are easily observed. During the earlier stages, which occur in 

 the Fallopian tubes, the ova are very small and difficult to find, but by the time' 

 the ovum has reached the uterus it has become a blastodermic vesicle measuring 

 about 0.6 mm. in diameter, and, therefore, easily seen by the naked eye. From 

 the fourth day after coitus until the beginning of the seventh day the vesicles lie 

 free in the uterus. Usually early in the seventh day the vesicles, which then 

 measure about 4.5 by 3.5 mm., begin to attach themselves to the wall of the 

 uterus, and thereafter are much more difficult to remove. At the beginning of the 

 fifth day the ova measure about 0.6 to 0.9 mm. in diameter, but vary greatly in 

 size, and are found more or less near together in the upper portion of the oviduct. 

 By the end of the sixth day they measure about 4.0 mm. and are distributed 

 throughout the entire length of the uterus. 



The most useful stages are the vesicles from the beginning of the sixth and 

 seventh days. To preserve the vesicles they must be gently removed from the 

 uterus, great care being necessary not to injure them, and dropped into Zenker's 

 or Hermann's fluid. In either of these they may be left for about an hour and 

 then washed and preserved in the usual manner. Specimens should be examined 

 in the fresh state, just after they have been preserved, and after they have been 

 stained, before they are imbedded. For staining, alum cochineal or borax carmine 



