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The sex of large mammals and most of the smaller ones can almost 
always be easily recognized by the external organs of animals in the flesh. 
With the larger mammals the sexual characters may frequently be noted 
on mutilated dried skins, or by the development of the skull, horns, or 
other appendages, but on account of warping and shrinkage of skins in 
drying or tanning, conditions are often distorted and difficult to observe 
with any degree of confidence, so the sex should always be determined 
from the fresh specimen and recorded. 
In small mammals the sexual characters are often small and obscure, 
particularly in young specimens in which the organs are undeveloped. In 
the first place, an animal which is cleanly killed, without blood clots, 
bruises, or discoloured organs is easier to examine. Torn viscera, intes- 
tinal juices, and decomposition will add to the other difficulties. If the 
animal is allowed to remain for some time, depending on the season, until 
the rigor mortis (stiffness of freshly killed animals) has passed off, there is 
not so much bother about blood flowing and obscuring the organs. 
Many of the smaller mammals have the sexual organs in both sexes 
greatly enlarged during the breeding season, and at such times the testes 
of adult males are generally conspicuous from the outside, and in cases 
of doubt may be easily verified by dissection. The nipples of nursing 
females are generally noticeably enlarged, and the fatty, whitish coloured 
mammary glands are conspicuous and diagnostic when the skin is removed. 
The nipples alone are not a perfect guide to the sex as they are frequently 
found in a vestigial state in the male of the species. 
In some mammals, particularly mice and shrews, during the intervals 
between the breeding seasons, and in young specimens, the sexual organs 
are small, shrunken, or undeveloped, and the external genitalia are super- 
ficially much alike in the sexes. The male sheath, or tubular fold of skin 
into which the penis is retracted, looks much like the external orifice of 
the vagina. The organs are obscured by hairs, and frequently these part? 
are so close to the anal aperture that the structure cannot be differentiated 
without use of a magnifying glass. In such cases, the fold of skin should 
be picked up with fine-pointed forceps and manipulated or forced back, 
and in the case of a male the organ will protrude, looking like a small 
-white thread. In some cases ossification may be detected, but the organ 
is so slender that the condition is hard to observe in gross dissection. If 
still in doubt, the presence or absence of the testes should be verified as 
the animal is being skinned. The testes are whitish or yellowish organs 
when not discoloured by ingested blood, and in a shrew may not be larger 
than a grain of sand. Care should be taken not to confuse the testes with 
grains of cornmeal or sawdust used in the skinning operations. The use 
of a small hand lens or magnifying glass is necessary in some cases, and 
should be a part of every collector’s and naturalist’s equipment. 
The condition of the reproductive organs — state of testes, whether 
small, slightly enlarged, or greatly enlarged — may be shown by an outline 
sketch drawn on back of the label; whether testes are descended or not 
descended; condition of the mammae (lactating) ; presence or absence of 
embryos. If embryos are present, their number and size should be 
indicated. 
The smaller mammals frequently have several litters of young during 
the warm season in the North, or at any time of the year in more southern 
