DEVELOPMENT OF THE ALBINO RAT 7 



of observed coitus. The slight though observable variation in 

 the rate of development in a series of ova of the same animal, 

 more marked when supposedly similar stages of several animals 

 are investigated, precludes the accurate timing of stages. 



As fixing fluids, there were used Zenker's fluid, sublimate- 

 alcohol, Flemming's fluid, Bourn's fluid, and Carnoy's fluid. 

 After a few trials, all were discarded in favor of Carnoy's fluid, 

 prepared by mixing 6 parts of absolute alcohol, 3 parts of chloro- 

 form, and 1 part of glacial acetic acid. This somewhat illogi- 

 cally compounded fluid penetrates rapidly and does not cause 

 shrinkage. Tissues are fixed in it for several hours, then washed 

 in several changes of absolute alcohol in which it has been my 

 custom to store the tissues. The following procedure was prac- 

 ticed in all stages up to about 12 days after insemination: The 

 animals were anaesthetized and the head severed from the body, 

 to admit of free bleeding. The rat was then fastened to a 

 board, and thorax and abdomen opened by a mid-sagittal in- 

 cision, the abdominal walls pinned back, and the intestine ele- 

 vated toward the thorax. With as little manipulation as pos- 

 sible, the ovaries were separated from their attachment, the 

 mesometrium cut, the uterine horns elevated and the vagina 

 severed. The whole genital tract was then placed on a clean 

 slide and arranged in approximately normal position. Slight 

 tension was maintained by tying a thread to the connective 

 tissue removed with each ovary and bringing the threads along 

 the reverse side of the slide and tying them to the vagina. If 

 the slide is clean, the mesometrium of each uterine horn may be 

 spread out evenly and caused to adhere to the slide. Ovaries, 

 oviducts, and uterine horns may thus be spread out in normal 

 position and each uterine horn fixed as a straight tube. When 

 thus arranged on the slide, the preparation was placed in a 

 relatively large quantity of Carnoy's fluid, fixed, and then trans- 

 ferred through several absolute alcohols. For nearly all the 

 material used in this study, the method of fixation was as here 

 given. In the earlier stages of material collection, attempts 

 were made to obtain segmentation stages in warm normal salt 

 solution. Several were thus obtained and were used to control 



