124 CYCLIC CHANGES IN THE OVARIES AND UTERUS OF THE SOW, ETC. 



The washing-out method is very satisfactory for recovering ova and vesicular 

 embryos from the uterus, but if it is inadvertently applied to a specimen containing 

 embryos of the latter part of the second week or of the third week, which have 

 undergone the extraordinary extension in length of the chorion which is character- 

 istic of the pig and some other ungulate species, the result will be an almost hopeless 

 tangle of chorionic membranes. Therefore, it is a wise precaution, when expecting 

 early embryos of uncertain age, to slit up the cervical end of the uterus for a distance 

 of several centimeters, watching for the delicate, glutinous, threadlike appearance 

 which marks the early chorion. If none is found, then any still earlier embryos 

 which may be present are presumably grouped somewhere in the remaining part of 

 the uterus and may be recovered by washing. 



Embryos of the third week and older have been removed by the usual method 

 employed by embryologists, namely, by cautious opening of the uterus under salt 

 solution in a well-illuminated glass dish against a dark background. The salt 

 solution used in the work has generally been prepared according to the formula of 

 Locke, except for the omission of glucose. 



The large size of the mature Graafian follicle also presents difficulty when it is 

 desired to locate the undischarged ovum. The use of serial sections of the whole 

 follicle is sometimes necessary, in which case much labor can often be saved by 

 embedding in celloidin and employing an assistant to cut the sections, which the 

 observer examines in turn as they are cut, until the cumulus oophorus begins to 

 appear in the sections, after which still greater care is used until the ovum itself is 

 reached. Another method is to lay the hardened ovary into serial slices of about 1 

 mm. thickness and then to search the follicle walls in the slices under the binocular 

 microscope until the cumuli are found. The ovum-bearing portion of the follicle is 

 cut out of the slice and sectioned in paraffin. As there are usually several follicles 

 in each ovary, the writer's practice has been to try the latter method, reserving 

 enough of the follicles to assure success by the former plan if the latter fails. 



Histological preparations have been made by the usual methods. Bouin's 

 picro-acetic-formol has been the chief fixing fluid, supplemented, of course, by a 

 variety of others for special purposes. The stains used will be specified in con- 

 nection with the illustrations. When it was necessary to wash out the uterus, a 

 block for histological preservation was first cut from the middle of one cornu in 

 order to spare the tissues any harm resulting from the fluid and from the pressure; 

 the portions remaining were then separately washed out for the ova. 



As it is difficult to cut thin sections of the sow's uterus except from very small 

 blocks, complete transverse sections of each uterus were made by the celloidin 

 technique to serve as reference for the small blocks used for study of cytological and 

 microchemical details. 



The writer's thanks are due to the American Feeding Company and to Hohman 

 & Sons of Baltimore, to the Western Meat Company of San Francisco, and to 

 Joseph Stern & Company of New York City for opportunity to collect this 

 material; and to the Department of Embryology of the Carnegie Institution for 

 technical assistance. 



