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The Laboratory Mouse. Its Origin, Heredity 
and Culture. Clyde E. Keeler, 81 pp. Harvard 
University Press. 193 1. 

A brief statement of the geographical distribu- 
tion of the mouse is followed by an informing 
account of the antiquity of the fancy mouse. It 
appears that dominant spotting, albinism, and 
waltzing were all recorded before the present era. 
The other breeds were distinguished much later. 
Since in the classical literature the same word 
Mns was used for both the mouse and the rat, it 
is only possible to determine which is meant by 
the help of indirect evidence. 

Some twenty-four breeds of mice are briefly 
described and these descriptions followed by a 
useful table, listing for twenty-seven varieties the 
fanciers' term, the scientific term and the genetic 

The genetics of normal and abnormal inherit- 
ance are then considered, and the book closes with 
a chapter on the laboratory breeding and care of 
these animals. 

Twenty-one life-size figures, in black and white, 
indicate the habit and coat color in the several 
breeds. — H. H. Donaldson. 

Reviewed in the "Collecting Net" 
August 8, 1931, by Dr. H. H. 
Donaldson, and presented to the 
Library or the Marine Biological 
Labora tory 









Cambridge Massachusetts 


















I. Introduction 3 

II. Geographical Distribution of the House Mouse ... 4 

III. Antiquity of the Fancy Mouse 7 

IV. Unit-Characters (Gene Mutations) of the House Mouse 19 
V. Normal Inheritance 35 

VI. Abnormal Inheritance 44 

VII. The Breeding of Mice in Laboratories 47 

Bibliography 73 




Fig. 1. The Egyptian Cat-goddess, Bubastis 8 

2. Polychrome pottery mouse from Egypt 8 

3. A. A coin of Alexandria Troas bearing the cultus statue of 

Apollo Smintheus 10 

B. A coin of Tenedos (300 B.C.) bearing the statue of Apollo 

Smintheus and a mouse 10 

4. The Japanese God of Wealth, Dai-koku, and his symbolic 

white mouse 15 

5. The mouse netsuke by the Japanese artist, Masateru ... 19 

6. Diagram illustrating the inheritance of a simple, recessive, 

mendelizing unit-character such as albinism 36 

7. Diagram of a pair of chromosomes showing the linkage re- 

lationship of two pairs of genes before, during, and after a 

crossing-over 41 

8. Diagram of a back-cross illustrating the linkage between rod- 

less retina and silver pelage 42 

9. Diagram of chromosomes of the house mouse with genes 

for unit characters distributed arbitrarily among them to 

show genetic independence or linkage of the characters . 43 

10. Wire mouse cage used at the Bussey Institution 53 

11. Sectional view of feeding can 57 

12. Gray (wild coated) 59 

13. Brown extreme dilute (bb c d c d ) 59 

14. Cinnamon chinchilla (bb c ch c ch ) 59 

15. Albino (cc) 59 

16. Lethal yellow (A } 'A) .' 61 

17. Non-agouti black (aa) 61 

18. Sooty yellow (sable) (A r a) 61 

19. Black-and-tan or white-bellied non-agouti black (at at) . . 61 

20. Gray recessive spotted (piebald) (ss) 63 

21. Non-agouti black with dominant spotting (aa Ww) .... 63 

22. Japanese waltzer non-agouti black piebald (selected for 

whiteness) (aa ss vv) 63 

23. Non-agouti black-eyed-white (homozygous for piebald) (aa 

Ww ss) 63 

24. Blue or non-agouti dilute black (aa dd) 65 

25. Non-agouti silver (aa ss) ^5 

26. Non-agouti brown (aa bb) 65 



Fig. 27. Short-ear lilac or pink-eye non-agouti black short-ear (aa 

pp se se) 65 

28. Heterozygous naked ("nakt") (N?i) 67 

29. Recessive hairless (hr hr) 67 

30. Homozygous naked (NN) 67 

31. Non-agouti dilute brown (aa bb dd) having developed two 

large transplanted tumors 67 

32. New-born mouse showing posterior reduplication. (Courtesy 

of Dr. C. H. Danforth) 69 

33. Non-agouti silver, normal and dwarf (2 1-2 months old (aa 

si si) and (aa si si dw dtv) 69 

34. Albino showing flexed tail 69 

35. Skulls of mice showing normal and parted f rentals .... 69 

36. Normal and rodless (rr) retinae of the house mouse ... 69 




Small rodents will always find a place in the laboratory of 
the zoology teacher, the biological investigator, the medical 
researcher, and the fancier. Each man has different problems 
in mind: behavior, physiology, disease, and beauty among 

On account of certain innate qualities the house mouse, 
Mus musculus, has become in many ways the laboratory 
mammal most favorable for culturing. Its fertility, prolific- 
ity, convenient size, short gestation period, its manifold 
variations, inexpensive maintenance, resistance to open in- 
fections, susceptibility to certain diseases, and ease of produc- 
tion conspire to make it the laboratory animal par excellence. 
The teacher of zoology uses variations of the house mouse 
to demonstrate the laws of heredity, the biological investi- 
gator employs them for physiological and genetic studies, 
the advanced medical man uses them as media in which to 
culture disease germs or for pathological tests as in the pro- 
duction of sera, and the fancier prizes them for their aesthetic 

Literature upon the house mouse, its origin, history, 
distribution, development, the nature of its variations, 
the hereditary transmission of its varietal characters, and 
methods of rearing it suitable for the needs of laboratories, 
has not been assembled so far as I am aware. . The data are 
in some instances rare, usually widely scattered, and often 
inaccessible to those who could advantageously employ 
them. Some are recorded in difficult and highly technical 
language. Some of the data have never been published. 

To collect such valuable information as this concern- 
ing the house mouse and to present it in a usable form is 
the task of this book. 



The great order of Rodents or gnawing mammals is very 
successful as judged by the extent of its distribution and the 
degree of its adaptation to varied environments. Cavies 
scuttle under brush, rats slink about human habitations, 
mice squeeze through inconceivably small holes, squirrels 
scurry up trees and leap or glide from branch to branch, 
rabbits tunnel the earth, amphibious beavers fell trees and 
build dams. Yet all are hopelessly dependent upon their 
chisel-like incisors, which proclaim a common relationship 
and give them a common name. 

The five families of rodents enjoying the widest distribu- 
tion (7) l are the Leporidse (rabbits, hares), the Hystricidse 
(porcupines), Sciuridse (squirrels), Cricetidae (New World 
mice, meadow mice, hamsters), and the Muridae (Old World 
rats and mice having tubercular teeth). 

Because the rabbits have four incisors in the upper jaw 
and two in the lower, they have been assigned to the sub- 
order Duplicidentata (duplex-toothed) or even made a 
separate order, Lagomorpha. The porcupines, squirrels, 
rats, and mice bear two incisors both above and below and 
are placed in the sub-order Simplicidentata (simple-toothed). 

Of these successful families the last two have attained 
world distribution, while the other three had established 
themselves before modern times in all geographical regions 
except the Australian. 

The accepted classification of the common house mouse is : 

Order Rodentia (gnawing animals) 

Sub-order Simplicidentata (simple-toothed) 
Family Muridre (mouse-like animals) 
Genus Mus (true mice) 

Species musculus (the little mouse) 

1 Italic figures in parentheses refer to names listed numerically in the Bibliog- 
raphy at the end of the text. 



The ancestral house mouse, from the present-day species 
of which the main breeds of the fancy are derived, is un- 
doubtedly of Central Asiatic origin. The date of its first 
appearance in Eocene times and its subsequent expansion 
may only be conjectured in terms of numerous millennia. 
From the region of its early development it made its way to 
the habitable portions of Europe, Asia, and northern Africa, 
perhaps often as a stowaway in early human migrations. 
At the present time the species Mus musculus is represented 
in the old world by several interbreeding species and their 
natural varieties. In northern Africa and Syria the pale, 
white-bellied M. musculus gentilis is found. M. musculus of 
southeastern Europe is in general darker than that of north- 
ern Europe, and it seems to be the darker animal that be- 
came established in Mexico and South America through 
colonization by South Europeans, whereas the lighter form 
is more common in the United States and Canada. 

The Asiatic Mus bactrianus group is lighter and more deli- 
cate than the European M . musculus, but breeds freely with 
it. Fancy breeds of mice often partake of blood of both 
types, while the Japanese waltzing mouse of the fancy may 
be derived (158, 59) solely from the bactrianus of China or 

Mus bactrianus (or wagneri (55)) ranges from Persia to 
China. The general type of bactrianus blends into the short- 
tailed M. bactrianus gansuensis of Mongolia and into the 
long-tailed M. bactrianus kakhycncnsis in Indo-China and 
the Malay peninsula (4). 

The species Mus musculus proper shared with the Euro- 
pean his recent conquest of the globe, and was uninten- 
tionally transported on ships or among merchandise to all 
habitable regions including the Asiatic seacoast normally 
within the range of the bactrianus group. 

This internationalization of the mouse has been so recent, 
and complete isolation has been so rare, that few distinct 
varieties have been able to develop and persist as such in 
nature. However, where some isolation has been afforded, 


several color varieties have maintained themselves distinct 
for many years. For example, a white-bellied colony of mice 
has been found near Woods Hole, Massachusetts (13Jf.) ; in 
a region in northern France the black (non-agouti) form has 
become the sole house mouse to the exclusion of the normal 
gray variety; the Isle of Wight off the coast of England is 
populated with the pink-eyed variation. 

The present universal supremacy of M. musculus or 
M. bactrianus as house mouse is challenged only in very 
circumscribed localities where the white-footed field mouse 
occasionally supplants it. The largest such area is thought 
to be southern Persia, where Apodemus sylvaticus takes its 



Variations of the word "mouse" found today in many 
European languages go back through the Latin mus and 
Greek mus or mys to mush in Sanscrit (100), the mother 
tongue of the race. In Sanscrit mush is derived from a verb 
meaning "to steal." This suggests that man was well 
acquainted with the mouse and its predatory habits before 
the separation of the Aryan tribes in Asia some four thou- 
sand years before Christ. One of the old Zoroastrian legends 
says that the moon chases away the clouds as a cat (weasel?) 
chases mice. 

Rats and mice abounded in the ancient world from earliest 
times, especially where grain was stored. There are numer- 
ous historical accounts of excessive increase of these rodents 
constituting veritable plagues in ancient civilizations, some- 
times accompanied by disease (138). 

Stories of rats and mice became early incorporated in 
the folklore of the ancient world along with anecdotes about 
their enemies the cat and the weasel. A specific word for 
house mouse as well as legends concerning it exist today in 
nearly every human dialect. 

In Egypt. The fact that rats and mice appear so rarely 
in Egyptian art is probably due to the fact that they were 
considered undesirable animals, and for this same reason 
the Nubian cat, as a destroyer of rats and mice, was deified 
before the Third Dynasty (c. 2800 B.C.). 

The sacred cat Bubastis (see Fig. 1) was not only the 
patron goddess of the prosperous delta city of Bast, but also 
the goddess of love and feminine fashion. Iphthimis her son 
was the god of goodness. In the holy city of Bast stood the 
famous cat mausoleum, where the remains of sacred felines 
were ceremoniously laid to rest in bronze or wooden cat- 



shaped coffins. Each year Lower Egypt thronged to the 
riotous feast of Bubastis, and families went through a mourn- 
ing ritual for their deceased cats similar to that 
for human members of the household. Since 
the cat embodied all godly virtue, the mouse 
probably came to symbolize evil by contrast. 
Indeed, it may be suggested that rats and 
mice were probably responsible to a great de- 
gree for the cat's deification, because the Nile 
delta has been a grain-growing region since 
prehistoric times, and was undoubtedly over- 
run with these rodents before the advent of 
the cat from Nubia. 

A glazed polychrome effigy of a white- 
bellied agouti mouse made in Egypt 2000 
B.C. is in the British Museum (see Fig. 2). 

A satyrical papyrus of the New Kingdom 
(153) (written between 1580-1205 B.C.) bears 
the picture of a rat or mouse (possibly Mus 
cdexandrinus) in kingly robes, attended by Egyptian cats. 

Aelianus (c. a.d. 100) remarks that in Lower Egypt mice 
develop from raindrops. St. Basil (a.d. 330-379) repeats the 
story of pluvial generation of mice in Egypt, but adds grass- 
hoppers and frogs as co-creations. 

Fig. 1. The Egyptian 
Cat-goddess, Bubastis, 
redrawn from Keller 
after Perrot-Chipiez. 

Fig. 2. Polychrome pottery mouse from Egypt, c. <2000 B.C. 
(In British Museum.) 

In Palestine. Moses received his cultural training in Egypt 
and with it the traditional hatred for mice. This attitude 
is exemplified among the commandments to the Hebrews 
recorded in the Book of Leviticus: 


These also shall be an abomination to you among the creeping things 
that creep upon the earth; the weasel, and the mouse, and the tortoise after 
his kind. . . . These are unclean to you among all that creep: whosoever 
doth touch them, when they be dead, shall be unclean until even. 

In the Old Testament is an account of a rat or mouse 
plague accompanied by intestinal disease which was brought 
upon the Philistines about 1000 B.C. following the seizure 
of the Israelitish "ark of the covenant." Golden effigies 
both of mice and of the affected portions of the human 
anatomy were given as gifts to Y'ahweh in order to appease 
his wrath. 1 

In Asia Minor. Probably the first recorded instance of 
the raising and protection of mice by men is in connection 
with the ancient mouse worship of Pontis instituted perhaps 
some fourteen hundred years before Christ. Homeric legend 
(c.1200 B.C.) mentions Apollo Smintheus (god of mice). This 
cult was popular at the time of Alexander the Great (300 


During the latter part of the second millennium before 
Christ, Cretan Teucri invaders landed upon the shore of 
Asia Minor for the purpose of colonization. For a long time 
they were restricted to the coast by the aboriginal Pontians, 
with whom they continuously contested in arms. A decisive 
victory for the Cretans was credited to the mice (probably 
field mice), which their Apollo caused to gnaw the leather 
straps from the shields of the enemy (65). 

Several Greek and Roman historians (1, 163) describe in 
some detail the temple which the conquering Teucri erected 
upon the Pontic island of Tenedos in gratitude to Apollo, 
god of mice. Tradition has it that before the Teucri set out 
from Crete they had been given an oracle commanding them 
that where they settled there they should build a temple 

1 Herakles Kornopion, the Tyrian Sun god as well as Baalzebub (harmonized 
by the Greeks as the Fly god Zeus) frequently bears the mouse symbol. On a 
Carthaginian votive stone described by Vigouroux-Ibach are carved two mice as 
gifts to either Baal or Astarte-Aphrodite. Herodotus says that the statue of Seti 
III within the temple of Ptah at Memphis had a mouse on the hand and bore the 
inscription: "Look on me and be just!" (100) 


to Apollo and worship the "earthborn creatures." The 
foundation of this temple was still standing in 1902. The ac- 
counts picture vividly a magnificent marble shrine overrun 
with sacred mice which were raised at public expense. They 
describe the altar, tripod, and statue of "Apollo, God of 
Mice." He stands stiffly in the style of the archaic Greek 
period. In his right hand he holds a patera, in his left he 

Fig. 3. 

A. A coin of Alexandria Troas bearing the cultus statue of Apollo Smintheus. (Natural size.) 

B. A coin of Tenedos (300 b.c.) bearing the statue of Apollo Smintheus and a mouse. 
(Enlarged about 3 diameters.) 

carries a bow. At his feet is the huge effigy of a mouse, while 
a family of white mice have their nest under the altar itself 
(see Fig. 3). 

A priestess of this temple, Herophila by name, was said to 
have correctly interpreted Queen Hecuba's dream concern- 
ing the fall of Troy and the fate of herself and family. 

The mouse cult (56, 169) spread from Tenedos to Alex- 
andria, Hamaxitus, Larissaia, Parion, Heraclea, Grynaeus, 
and Chryse in Asia Minor, and the god was even honored in 
Lesbian Arisba, Methymna, and Magnesia. Record of the 
cult is to be found in Athens and Thespia on the Greek main- 
land. In some of these centers it probably continued as a 
local form of worship until the Turkish conquest in 1453. 
Thus the Sminthian worship existed for about three thousand 
years and white mice were cultured in the temples for about 
two thirds of this period, mainly for auguries. 1 

1 Jamblichos is authority for the statement that mice were employed for augu- 
ries in Babylon, while Aelianus mentions the soothsaying rites of Apollo Smintheus 


Coins of the Troad, and especially Alexandria Troas, fre- 
quently bear the figure of the cultus statue and in several 
instances the sacred mouse (41) - 1 

Aristotle refers to the white mice of Pontis. Strabo (c. 25 
B.C.) (163) mentions the white mice cultured in the many 
Sminthian temples. Pliny (c. a.d. 25) (150) alludes to the 
use of white mice for auguries. White mice are mentioned 
by Aelianus (a.d. 100) (1), Hesychius (c. a.d. 500) (100), 
Suida (1100) (65), Albert (1250) (65), Apostolius (1453) 
(65), Gesner (1560) (65), Johnson (1640) (84), Pallas (1766) 
(143), and Darwin (1865) (36), while more recently the au- 
thorities writing upon them have been too numerous to 

Pliny says in this connection: 

And verily, how basely men thinke of this kind of eattell and hold them 
no better than vermine, yet are they not without eertaine naturall prop- 
erties, and those not to be despised : but principally in regard of the sym- 
pathy between them and the planets in their ascent, I have noted hereto- 
fore: and namely, considering how the lobes and filaments of their livers 
and bowels do increase or decrease in number according to the dais of the 
Moon's age. . . . By the learning of soothsayers, observed it is, that 
if there be a store of white ones bred it is a good signe and presageth 
prosperitie. — Translation of Philemon Holland, 1635. 

The pharmaceutical virtues of the mouse so often em- 
ployed by Greek and Christian doctors may be attributable 
in part to the influence of the mouse cult of Pontis. 

Greece and Rome. The Homeric story of " Batrachomyo- 
machia" or the "Battle of Frogs and Mice" probably origi- 
nated in Ionia about 750 b.c. An analagous story was 
popular in Europe during the early Christian centuries. The 
latter tale was known as Galliomyomachia ("The Battle 
of Weasels and Mice"), and describes a war waged by the 
weasels upon the rats. The story may have been occasioned 
by the influx into Europe of the black rat (Rattus rattus) 
following the migration of the Germanic tribes, which animal 
finally gained a foothold in England during the fifteenth 
century. . 

1 Coins of Nesos, Lampsakos, Nagidos, and Metapont bear a mouse. 


Aristotle (300 b.c.) said that mice were generated spon- 
taneously from filth in houses and in ships. Horace (65-8 
B.C.) wrote the famous story of the Country Mouse that 
returned a visit to his cousin the Town Mouse. Pliny (a.d. 
23-79) in his Historia Naturalis classified the different kinds 
of mice, calling the house mouse "musculus" (little mouse), 
which name it bears today in zoological taxonomy. Pliny 
also recorded that the ashes of a weasel sprinkled about the 
house will keep away mice. He stated that in Ionia mice 
are generated by the overflowing of the Meander River, 
even causing the inhabitants to flee from their dwellings. He 
told of the driving out of the entire population of the Isle of 
Gyarus by mice which proceeded to gnaw the gold, iron, and 
steel left behind. 

Europe. In Christian Europe mice and rats fell into dis- 
repute, becoming the companions of witches and sorcerers. 
This was partly due to the attitude of the Church following 
the condemnation of these creatures by Moses. 

The clergy of the Middle Ages never ceased to comment 
upon the voluptuous and libidinous habits of mice. Indeed, 
mice were frequently raised by curious churchmen in order 
to observe their wicked actions. Albert (65) records that the 
white ones are very lustful. In this statement he follows 
Diogenes. Gesner (65) says they are libidinous. Erasmus 
records their lust. 1 Part of the European attitude may have 
been due to plagues and current legends originating in Greek 
mythology. 2 

Horace (65 b.c.) spoke of the mountains being in labor and 
bringing forth a "ridiculus mus," which is to this day a 
proverb of futile effort. Aelianus calls them "earthborn 
creatures." 3 About a.d. 1500 Erasmus collected together 

1 The mouse was often a symbol of delicacy or lust in Greek drama. In the 
British Museum is a bronze mouse from Iconia which holds over its face the mask 
of a Silen, a creature usually represented as a man having a horse's tail, and the 
symbol of lust in the worship of Dionysos. 

2 The ancient Persians and Bactrians held that mice were creations of the 
wicked god Ahriman. 

3 A Talmudic fable speaks of a mouse in the process of creation, the fore parts 
already flesh and the hinder parts still earth. 


some eight hundred legends, a number of which were reputed 
to be the works of Aesop (c. 620-560 b.c). According to 
Erasmus this brand of murine-generation story was origi- 
nated by Aesop and copied by Porphyrion (a.d. 233-304), 
who gives the following account (65) : 

As once when wild and uncivilized men saw the earth to heave up and 
move in a mountain, they ran together from every direction to such a 
dreadful sight, expecting that the earth would there give forth some new 
and great spectacle (the mountain indisputably laboring). Perhaps it 
should be that the Titans would burst forth again and renew their war with 
the gods! Then, while all the multitude stood there in suspense with 
astonished spirits, a mouse broke out of the earth, and a laugh arose from 
all the people. 

Plutarch (a.d. 46-120) says that mice conceive by licking 
salt. He copies this from Aristotle, who not only believes in 
the saline method of engendering but records a ridiculous 
litter size of one hundred and twenty young produced 
through this kind of parthenogenesis. This story was brought 
to Aristotle by a veteran of Alexander's military campaign 
in India, who apparently wished to impress the old naturalist 
with the marvels of that far-away land. Thomas of Can- 
timpre (c. a.d. 1228-1244) avers that the size of mouse livers 
waxes and wanes with the moon, but in this he repeats Pliny. 

These legends expanded to their greatest proportions dur- 
ing the Middle Ages when mice along with other base 
creatures were considered the handiwork of devils. Casper 
Schott (a.d. 1697) (156) in his interesting Physica Curiosa 
is bold enough to suggest that diabolical assistance may not 
be necessary in the creation of lowly animals as commonly 
believed, because many of the forms are known to be pro- 
duced by spontaneous generation. He says: 

The first reason for doubting is because many animals arise from putrid 
material and by other means without the intervention of father and 
mother. . . . Indeed, agile boring larvae and little worms are given birth 
in rotting wood; from putrefaction slugs, snails and mice, from ox dung 
honey bee drones and wasps; from the aerated urine of caterpillars, butter- 
flies, ants, grasshoppers, cicadas and other similar- forms. 

In this tale he follows Pliny, who in turn copies it from 


The reputed parthenogenetic reproduction of the mouse 
was held up by the clergy as an example of the natural- 
ness of human parthenogenesis demanded by Christian the- 

Before the advent of the Persian cat into northern Europe 
during the time of Charlemagne, mice and rats frequently 
multiplied in such numbers that they could not be kept in 
check by the weasels maintained by the more fortunate 
families. These conditions gave rise to such stories as that 
of the Pied Piper of Hamlin. 

The early Greek and Roman physicians employed mice 
in their medicinal formulae. Hippocrates (300? B.C.) says that 
he did not test the virtue of mouse blood as a cure for warts, 
prescribed by his colleagues, because he had a magic stone 
with lumps upon it which had proved an efficient remedy. 
Galen (a.d. 130?- c 200?) advocates equal parts of mouse 
blood, cock's gall, and woman's milk mixed and dried as a 
cure for cataract. Villanova uses dog's urine and mouse 
blood for warts. 

During the Dark Ages the formulae became increasingly 
occult and complicated, and mice figured even more in the 
pharmacopoeia. 1 St. Hildegarde of Bingen (1098-1179) re- 
counts that mice are a cure for epilepsy. The manuscript 
known under the name of Picatrix (l l 2o6) endorses fumigation 
with fourteen bats and twenty -four mice. Peter of Albano 
employs mouse dung as a cure for poisons, probably in- 
fluenced by Pliny's freshly killed mouse poultice for serpent 
bites. 2 

1 The mouse seems to disappear from medical formula? during the latter part of 
the seventeenth century, although crab claws and millipeds persist even in the 
literature of the last century. The London Pharmacopoeia {161/) of 1667 instructs 
as follows: "A flead mouse dried and beaten to powder, and given at a time, helps 
such as cannot hold their water or have Diabetes, if you do the like three daies 
together." The influence of this dictum was felt at Boston, Massachusetts, as late 
as 1890, when a family of English extraction fed mouse stew to their children to 
prevent bed-wetting. 

2 In Europe mice used to be eaten as a remedy for toothache. New-born mice 
dissolved in olive oil are a popular panacea for human ills in Turkey and Greece 



The Orient. Although until recently the house mouse has 
been openly despised by Christian teachers, in the Orient, 
on the contrary, it has always enjoyed a much higher social 

Albino mice were used by the Chinese priests for auguries 
and during many centuries the government preserved records 
of their taking in the wild. These records cover the period 

Fig. 4. The Japanese God of Wealth, Dal-koku, and his symbolic white mouse. 
(After a print in the Museum of Fine Arts in Boston.) 

between a.d. 307 and a. d. 1641. During this time the finding 
of about thirty albino mice was recorded by the magistrates. 

From Turkestan to Japan, years are reckoned in cycles of 
twelve, the first year of each cycle being named the "Mouse." 

In Japan the mouse of the folk-sagas is a very wise creature 
and the symbol and messenger of the God of ^Yealth, Da'i- 
koku (see Fig. 4). The god is usually represented as stand- 
ing upon two sacks of rice with a mouse perched at his feet. 
The time between 11 p.m. and 1 a.m. is known as the hour of 
the mouse. A children's story describes the wedding of the 


mice. 1 Thus national tradition provided a psychological 
attitude among the Japanese most favorable for the develop- 
ment of the mouse as a fancy animal. In Japan today the 
mouse fancy is well developed, having thriven for at least 
three centuries. 

It is difficult to ascertain how long varieties of the house 
mouse have been recognized in China. The word for white 
mouse is ancient, and that for spotted mouse appears in the 
earliest Chinese lexicon, written 1100 B.C. The waltzing 
variety has been known since 80 b.c. That the Nipponese of 
Yokohama and elsewhere zealously collected new varieties 
in foreign lands is shown by the fact that they call a mouse 
bearing certain markings the "Nanking Mouse" (162), 
while the Chinese fanciers of Shanghai near Nanking deny its 
origination and call it the foreign mouse. The Japanese 
waltzer was undoubtedly derived, at least in part, from Mus 
bactrianus (wagneri) of Tibet, as pointed out by Bowdler 
Sharpe (158) in 1912. Moreover, Mus musculus proper is not 
native to Japan. Perhaps the Japanese procured the Euro- 
pean M. musculus varieties from Portuguese traders. 

The Japanese had in their fancy such varietal characteris- 
tics as albinism, non-agouti, chocolate, waltzing, dominant 
and recessive spotting, and possibly blue dilution, pink- 
eyed dilution, and lethal yellow. 

Something over a hundred years ago several of these 
fancy varieties of the house mouse were taken from Japan 
to Europe by British traders, and only a few decades ago did 
muriculture spread to America. 

During the nineteenth century a number of European 
zoologists bred fancy mice for scientific investigation of the 
inheritance of varietal characters. They accumulated valu- 
able information, but the meaning of these data remained 
unknown until the rediscovery of Mendel's Law of Heredity 
in 1900 (33). 

1 The Japanese have a saying that white mice are good and honest while dark 
ones are wicked and dishonest. Believing that good overcomes evil, some Japanese 
bring white mice into their houses in order to drive out the wild gray ones. 



The essential feature of this law is the fact that the charac- 
teristics which differentiate domestic varieties are inherited 
as units, capable of being combined in all possible ways 
through the agency of hybridization. Up to the present time 
about two dozen such unit-characters have been recorded 
for the house mouse. The more important of these appear 
to have been first recorded at dates approximately as follows : 





Dominant spotting c.1100 

Albinism 300 

Waltzing 80 


Pink-eye dilution 1640 

Black 1640 

Recessive spotting (piebald) 1766 

Chocolate 1843 

Naked (dominant hairless) 1850 

Chinchilla c.1890 

Extreme dilution c.1890 

Yellow 1902 

Blue 1903 

Short ears 1921 

Rodless 1924 

Recessive hairless 1926 

Shaker 1926 

Hyperglycemia 1926 

Dwarf 1929 

Eh Yah 


Annals of Han Dynasty 







"An old fancier" 






Lord and Gates 

Cammidge and Howard 


In accordance with our present ideas, each unit-character 
made its appearance as a sudden, discontinuous physical or 
chemical change in the germinal substance, which forms the 
basis of heredity. We call such changes mutations, and the 
material bodies in which these changes occur are called 
genes. We can demonstrate the existence of a gene only 
when, as a consequence of mutation, it occurs in two differ- 
ent alternative forms in different individuals of the same 
species. By crossing individuals which bear different allelo- 
morphs of the same gene, we can show that transmission of 
the contrasted characters conforms with Mendel's Law. 


Each unit-character is borne in or determined by a differ- 
ent gene, and is independent in its transmission of every 
other gene, except such as lie in the same chromosome with 
itself, a complication resulting in genetic linkage, a phenome- 
non to be more fully discussed later. 



The Japanese, as already stated, must be given credit for 
the development of a number of varieties of domestic mice. 
An ivory netsuke or sash pendant in the Louvre {155) carved 
about 1790 by the Japanese artist Masateru depicts a family 
of fancy mice in natural color among which one may dis- 
tinguish the unit-characters pink-eye, piebald, non-agouti, 
albinism, and waltzing (see Fig. 5). Similar netsukes were 
popular during the nineteenth century. 

Fig. 5. The mouse aetsukl by the Japanese artist, Masateru. 
(After a photograph hy Schluruberger.) 

We have reason to believe that each unit-character arose 
by mutation or physical change in a particular gene located 
in a particular chromosome of a germ cell, and that this con- 
dition was transmitted to subsequent generations through 
heredity, the character manifesting itself in those individuals 
which carried certain hereditary combinations. 

We may also be confident that identical sports have arisen 
in the wild at different times in remote parts of the world. 
Among the stuffed skins of the house mouse in the British 
Museum collections in 1926 were found pink-eyed dilutes 
from the Isle of Wight off the coast of England and from 
Zanzibar (the same variety was recently taken in the wild in 
Germany). Albinos had been collected from numerous locali- 


ties. Several pied mice were listed from Syria. Blue-dilute 
specimens came from Esthonia and Syria, non-agouti (black) 
specimens from Cape Colony and England. Cinnamons were 
taken in South Africa and the Tigris Valley. There were 
white-bellied agoutis from Syria and Persia. Several speci- 
mens from West Africa probably contained extreme dilution. 
The mice were all taken in the wild and not purchased from 
fanciers. These facts refute the common belief that the 
varietal characteristics found in fancy stocks are the results 
of domestication. The natural cause which produces such 
mutations in the germ cells is as yet undiscovered. Under 
laboratory conditions /3-rays of radium and X-rays are able 
to produce in the fruit fly (Drosophila) mutations identical 
with those appearing in this species in nature (139). It is 
possible that cosmic rays in nature are responsible for the 
origin of some of the house-mouse variations. 

Myriads of mutations may arise and be lost in nature 
without ever being seen by man. Where accidentally a 
mutation has struck the fancy of man, he has secured and 
propagated it in captivity, selecting in the following genera- 
tions for the particular character in question. Where a 
mutant form arises in any one of the thousands of labora- 
tories breeding mice today {113, 71, 160, 146), it is quite 
likely to come to the attention of man and be preserved. 
Thus the number of mutants recorded as found in the wild 
is not comparable with the number observed to have origi- 
nated in laboratory stocks nor is it legitimate to conclude 
that the mutation rate in captivity is greater than that in 
the wild. 

Gray (normal wild coat). The gray coat (4 U 2) of a wild house 
mouse (see Fig. 12) is produced by the deposition of two 
kinds of pigment (yellow and black) in different portions of 
the hairs. Yellow is present normally in an apical or sub- 
apical band of many of the hairs. In general upon the 
ventral pelage the band becomes wider and the black pig- 
ment less, giving the belly a distinctly lighter appearance 
than the back. These pigment differences may be due to 


different types and rates of pigment production in the hair 
follicle, with an inhibitor for the black process when the 
banded region is forming. Chocolate pigmentation which 
takes the place of black in some fancy varieties may be 
looked upon as a condition in which the black reaction has 
been checked in an early phase. 

Albinism (c, mutant form of the color gene, C). 

The white mouse of Pontis is said to ruminate. — Aristotle, 300 B.C. 

Complete albinism (see Fig. 15) is a condition in which 
pigmentation is entirely wanting in all parts of the body. Not 
only are external organs devoid of pigment but even those 
internal regions which normally develop pigment, such as 
the eyeball and the outer surfaces of the brain and spinal 
cord, are unpigmented. Animals bearing albinism have 
white hair and pink skin. The eye color is usually either 
pink or whitish according to whether or not the retinal blood 
supply is visible through the iris. Histological examination 
reveals the fact that in albinos pigment granules are present 
which are normal in shape and distribution, but which are 

A common explanation (140) for the production of albi- 
nism is the absence of the colorless tyrosinase, which ferment, 
working upon the base or substrate, tyrosin, converts it into 
the pigment melanin. Several chemical reactions are in- 
volved in the process. Some maintain that it is the tyrosin 
which is lacking in albinos. This assumption appears more 
probable in view of the allelomorphic series of dilutions 
produced by different forms of the albino gene. 

The gene or hereditary determiner for albinism in the 
mouse is the lowest step in a series of alternative chemical 
states possible for this particular gene. The other states of 
this gene produce respectively normal pigmentation (C), 
chinchilla (c ch ), and extreme (Himalayan) dilution (c 11 ). 
A corresponding complete series of allelomorphs is found in 
rabbits. Albinism in crosses (18, 69) behaves as a recessive 


to normal pigmentation, chinchilla {157), and extreme dilu- 
tion (39). 

Complete albinism or one of its allelomorphs in which 
very little pigment is produced is known to occur in fish, 
birds, and most species of mammals, including man. 

Extreme Dilution (c H ) 

Mr. J. E. Knight brought into my laboratory a young male mutant 
mouse which he had captured in a corn crib. . . . This animal . . . gave 
the appearance of being an ordinary black eyed white in which the hair 
was apparently very slightly stained or dirty. — Detlefsen, 1921. 

The above description of the extreme-dilute mouse (see 
Fig. 13) is quite accurate (39). These animals vary in shade 
but are always a dirty white color. This is apparently due to 
a complete suppression of yellow and an almost complete 
suppression of black and brown in the coat. I have found 
that intense pigmentation persists in the ears, eyes, upon 
the tail, and to a lesser degree upon the feet. 

In the possession of pigmented extremities this mutant 
resembles Himalayan albinism of the rabbit, and indeed its 
determiner occupies a corresponding position among the 
alternative forms of the albino gene. 

Upon superficial examination of extreme dilutes it is im- 
possible to distinguish blacks, browns, agoutis, non-agoutis, 
and so forth. Closer inspection and continued handling en- 
ables one to differentiate blacks from browns by the shade 
of pigment upon the ears. These distinctions may be con- 
firmed by clearing the irida? in xylol. Such prepared iridse 
show normal black or chocolate pigmentation according to 
whether the animal is genetically a black or a brown. 1 

Chinchilla (c ch ) 

Thus the first generation of hybrids (between wild gray house mice 
and fancy albinos) consisted of 342 mice, of which 329 were gray, seven 
yellow and six chinchillas. — - Schuster, 1905. 

1 An old fancier in the fourth edition of Fancy Mice says: "Occasionally impure 
breeds and strains (of albinos) are raised in which there are black ears, eyes, and 
feet. . . ." This may indicate extreme dilution, which being a dominant allelomorph 
to true albinism might carry it as a recessive and continue to produce true albinos 
generation after generation. 


The chinchilla mouse with black agouti coat may be 
described (157) as a bluish gray containing no yellow, its 
pelage resembling closely that of the gray squirrel. Brown 
agouti animals containing the chinchilla factor (see Fig. 14) 
are easily distinguished by their brownish coat, with white 
rather than yellow bands of the agouti distribution pattern. 

Chinchilla has a tendency to remove yellow from the coat, 
although it also dilutes black and brown pigments, changing 
non-agouti black to sepia. This leaves parts of agouti hairs 
almost white and the bellies of agouti chinchilla animals are 
usually quite colorless. 

The yellow-reducing tendency is easily demonstrated by 
the fact that when lethal-yellow mice are crossed with 
chinchillas and yellow animals are produced carrying two 
doses of chinchilla, these animals have a cream coat and 
black eyes. 

Chinchilla (157, 57) of the mouse corresponds to chin- 
chilla of the rabbit both in appearance and genetic behavior, 
being in both cases produced by a form of the albino gene 
recessive to normal pigmentation and dominant to both 
extreme dilution (Himalayan) and true albinism. 1 

Pink-eyed dilution (p) 

Scaliger saw another (mouse) very bright, with flaming eyes. — John- 
son, 1640. 

This mutation (see Fig. 27) reduces greatly the black or 
brown pigment, giving the eye a beautiful pink tint, from the 
color of the blood in the eyeball. The coat of the agouti 
black is changed to a fawn. The coat of the agouti brown 
(cinnamon) becomes also a fawn but more brilliant than the 
pink-eyed black agouti, because brown pigment replaces 

1 Dr. C. Carter Blake (Fancy Mice, fourth edition) in a letter written about 
1890 describes two crosses in which albinos mated with albinos produced colored 
young. His results may be explained if it be assumed that the male used was really 
a synthetic albino homozygous for pink-eye, heterozygous for non-agouti, brown, 
and spotting, and bearing one dose each of chinchilla and albinism. If this be the 
true explanation of these crosses, then human experience with chinchilla antedates 
Schuster's crosses in which chinchilla came in heterozygously from a gray mouse 
caught in the wild. 


black. Pink-eyed dilution (32, 23, 19, 5Ji) changes non- 
agouti black to "lilac" and transforms chocolate to "cafe au 

That pink-eyed dilution works only upon black and brown 
pigment is demonstrated by the fact that the coat color of 
lethal yellow is almost unchanged by the addition of pink- 
eyed dilution although the black eye of the lethal yellow is 
changed to pink. 

Non-agouti (a) 

As to color, many are like the ass; however some are einerous, others 
even black, others from brown to red. — Johnson, ] 640. 

Non-agouti (see Fig. 17) is due to a mutation resulting in 
loss from the individual hairs of the normal banded distribu- 
tion pattern determined by the gene A. No yellow apical 
band is formed in the non-agouti animal, but the black or 
brown pigment extends the full length of the hair. Absence 
or inactivation of the agouti gene (A) in the mutant type 
non-agouti (53, 54) (aa), changes a gray mouse to black and 
a cinnamon to rich chocolate. 

Lethal yellow (A y ) 

In my cultures I found in addition yellow mice. ... — Cuenot, 1902. 

Presence of the yellow mutation (see Fig. 16) in a mouse 
is easily recognized by its brilliant yellow coat and jet-black 
(or brown) eye. These effects are due to the complete or 
nearly complete suppression of black pigment, save in the 
eye. In the eye of a yellow mouse black pigment is even 
more abundant than in that of a black mouse, as proven by 
clearing specimens of both types in xylol. 

Yellow is a dominant character, i.e. it requires but one 
dose of the gene to cause the animal to exhibit the character. 
Animals containing two doses of the yellow gene (homozy- 
gotes) are inviable (15, 24, 10^, 20, 79). Thus, as the yellow 
gene is an allelomorph to the agouti (166) (A) and non- 
agouti (a) genes, a yellow mouse will contain one dose of 
yellow and one of either agouti or non-agouti (A 1 A or A* a). 


Those yellows which contain a non-agouti gene (a) are 
often sooty yellow (see Fig. 18) in appearance, and have 
sometimes been called "sables" (47, 54)- 

Chocolate yellows are clearer in color than those contain- 
ing black, i.e. have a less sooty appearance. 

Reds (A Y A, or Y Y a plus darkening modifiers) 

These animals of the fancy are a dull red, similar in color 
to Rhode Island Red fowls. 

Genetically they are lethal yellows with intensifying modi- 
fiers. They breed as yellows, but the complete genetics of the 
modifiers has not yet been worked out, although Dunn (46) 
has studied it extensively and has shown that it must be 
considered as due to numerous modifying genes. 

Dominant spotting (IF) (broken spotting) 

A mouse with the hair pattern of a leopard. — Eh Yah, c. 1100 B.C. 

This variety (see Fig. 21) is characterized by the presence 
of small irregular broken spots or polka-dot patches of color 
upon a white ground. In higher grades pigmentation per- 
sists about the eyes and dorso-caudal region only. 

Dominant spotting (112, 162, 53, 72) is expressed, as the 
name indicates, in animals containing but one dose of the 
gene. Homozygotes die of anaemia at an early age, under 
eighteen days (37, 40). Fancy breeds of this type are known 
as black-eyed whites (see Fig. 23). They contain one domi- 
nant-spotting gene and two recessive-piebald genes, i.e. are 
heterozygous for dominant spotting and homozygous for 
recessive piebald. In some strains, perhaps all, pure-breds 
(homozygotes) are inviable. Dominant spotting may be 
due to localized inhibitors for the tyrosin-tyrosinase reaction. 
This type of spotting has come to us from Japan. In Chinese 
history it is recorded that in 120 b.c. (or more probably 
a.d. 40) a wise court official who was able to recall the name 
of this variety was rewarded by the Emperor with a cartload 
of silken textiles. 


Recessive spotting (s) ("piebald") 

I have seen indeed a gray variety with a white saddle and also a white 
varietj 7 spotted with black. — Pallas, 1766. 

In this type (see Fig. 20), large unbroken areas of white are 
present upon the belly, back, and face (3). There is a tend- 
ency to form a white belt and a white face. Recessive spot- 
ting (61) in its highest grade usually leaves two patches of 
pigment about the ears, the rest of the coat being white. 
This is the condition usually found in the Japanese waltzing 
mouse (see Fig. 22) . Additional pigmented spots when present 
are usually found upon the rump. Animals bearing but one 
dose of the higher grades of spotting often have a small 
belly patch of white, but are otherwise colored. It is possible 
by systematic selection to produce strains with white faces 
or belts (50). What has been said concerning the probable 
immediate cause of dominant spotting (tyrosintyrosinase 
inhibition) applies to recessive spotting as well. 

Yellow belly (A w , a 1 ) (white belly, light belly) 

The back is a gray tinged with red-brown, the belly is bordered with 
red-brown: the exact livery of the field mouse. — Cuenot, 1907. 

It has been found in the laboratory as well as in the wild 
state that mice may mutate (113, 136) to a yellow-bellied or 
white-bellied condition (see Fig. 19), in which the ventral 
hairs have an exceedingly long, light, apical band, in some 
cases even to the exclusion of all other pigment. There is a 
characteristically sharp demarcation on the sides and under 
the chin, with a patch of darkly pigmented hair upon the 
neck, shaped like a bow tie. 

This bodily distribution pattern behaves as a dominant 
to all known color characters or their combinations, includ- 
ing yellow, i.e. it may be associated with the general coat 
coloration of any other type. It has been found in agouti 
(23, 137) and non-agouti forms (lJf.6), but not in combina- 
tion with the lethal-yellow mutation. The wild gray mouse 
possessing this character has a normal back and a white or 
buff belly often tinged with red-brown along the sides (A w ). 


A black mouse bearing this pattern is known as black-and-tan 
(a 1 ) {51). The chocolate mouse with this pattern is a choco- 
late-and-buff. The genetic relations of A w to a 1 are uncer- 
tain, except that they behave as allelomorphs of each other 
and of the agouti gene A. 

When a gray mouse is crossed with a black-and-tan, the 
offspring are light-bellied grays. Progeny of identical appear- 
ance may be produced when white-bellied grays are crossed 
with blacks. Black-and-tan and white-bellied gray have 
usually been regarded as two additional allelomorphs of 
the yellow, agouti, non-agouti series (51, 23). But it would 
seem to be a more probable explanation that white belly 
depends upon a gene closely linked with the agouti gene. 
This explanation eliminates the paradox of the black-and- 
tan, which, when crossed to gray, produces offspring in 
which the back behaves as a recessive and the belly as a 

Brown (b) 

The first definite record of the brown mouse is found in a specimen list 
of mammals in the British Museum by Gray, 1843. 

The brown condition (see Fig. 26) is one in which black 
pigment throughout the coat, skin, and eyes is replaced by 
chocolate (54, 110). This is thought to be due to an early 
interruption of the reaction which regularly produces black 
pigment. The mouse which would otherwise be wild gray 
coated becomes a cinnamon when homozygous for the 
brown gene, and the unticked or non-agouti form becomes 
pure chocolate in color. 

Blue dilution (d) 

Blues may be thrown by blacks and then breed true. — Bateson, 1903. 

A gray mouse homozygous for blue dilution (9) has a coat 
exhibiting a washed-out appearance known as blue-gray 
(53, 51f,, 72, 110). A non-agouti black when homozygous 
for blue dilution (see Fig. 24) becomes lead colored like a 
Maltese cat. A chocolate mouse which is also dilute is of a 


''silver-fawn" color. The blue-dilute condition consists in a 
reduction in number of pigment granules in addition to a 
clumping of these granules. There is no noticeable change 
in iris pigmentation, but the cleared retina is lighter in color 
than that of the normal black eye. 

Naked (N) (dominant hairless, "half-naked") 

The whole bodies of these three little creatures were completely naked, 
as destitute of hair and as fair as a child's cheek. There was nothing 
peculiar about the snout, whiskers, ears, lower half of the legs and tail: 
all of which had hair of the usual length and colour. — Gordon, 1850. 

Naked (see Fig. 28) is a peculiar physiological condition of 
the skin causing alternate waves of falling hair and regenera- 
tion (66). These waves (108) pass from head to tail, three 
or four waves being visible at a given time. Vibrissa? are 
present as well as the short hairs upon the tail. The 
homozygous naked (see Fig. 30) is devoid of tail hair and 
vibrissa?, and is semi-lethal. Animals of this constitution are 
difficult to raise and are usually sterile. These mice have the 
skin normal in texture and general appearance. The genetics 
of this variation as a dominant unit-character was first 
worked out by Lebedinsky and Dauvart (1927). 

Recessive hairless (hr) (rhinocerus?) (151, 3, 14) 

In November, 1924, I received from a gentleman in North London, a 
pair of pink, smooth-skinned, hairless mice, which he had captured in his 
aviary. — Brooke, 19 L 26. 

This type of hairless animal (see Fig. 29) has no tail hair 
but retains vibrissa? (12, 130, 60). It has wrinkled, dry skin 
of at least three times normal thickness. A few aberrant 
hairs coil about within and even under the skin. Such hairs 
may be seen in the dried skin, which is rendered transparent 
by the natural oil contained within it. The skin is filled with 
granular cysts. Hairless females are usually sterile and the 
stock is maintained by breeding hairless males to their heter- 
ozygous sisters. The gene for hairless (hr) is linked with 
piebald (s). 


Short ears (se) 

The mutation was found in a stock which originally came from the 
Lathrop mouse farm and consists in a noticeable difference in size of ears. 
— Lynch, 1921. 

As the name would indicate, this variety is characterized 
by small ears (see Fig. 27). It is due to arrested development 
of the auditory pinna {127). When both short-eared and 
normal-eared animals are found in the same litter, one is 
often able to distinguish the classes upon the fourteenth day 
after birth. 

While the normal ear increases in length of pinna (87) 
during the time from the fourteenth to the twenty-eighth 
day from 0.71 to 1.16 cm. (63 per cent increase), the short 
ear shows a gain from .60 to .76 cm. (27 per cent increase). 
The skull shape of the short-eared mouse differs from that of 
normals in that the nose is broader, while the cranium is 
much narrower. The zygomatic arch is squarer than in the 
normal. There is a certain amount of sterility noticeable 
among short-ear animals. The gene underlying the develop- 
ment of short ears is tightly linked with that for blue dilu- 
tion and there is a relationship, not completely worked out, 
between short ears and wavy tail. This latter relationship 
will be touched upon under the heading of wavy tail. 

Wavy tail 

The behavior of this mutation is, like tailless, eccentric. — Gates, 1927. 

Some mice from birth show in the tail a series of zigzag 
waves (61) bent in the horizontal plane. Such a condition is 
commonly found in short-ear stocks. Wavy tail is purely 
a neuromuscular condition (87), more extreme when the 
animal is excited and disappearing in sickness, death, or 
under anaesthesia. The loci of the flexures are constant 
throughout life, as may be shown by tattooing a spot at the 
point of each flexure. X-ray photographs reveal that the 
caudal vertebrae are unaffected. The extended wavy tail is of 
normal length. Snell maintains (159) that wavy tail is but a 
second expression of the short-ear gene. Yet in some short- 


ear families wavy tail appears to be absent. In some long-ear 
stocks it is present. In the absence of complete genetic 
analysis it is impossible to say whether the short-ear stocks 
with straight tails lack the wavy tail through absence of a 
gene or merely through lack of muscle tonus. If the wavy 
tail is not another expression of the short-ear gene, then it 
is linked with short ears as the researches of Gates and Keeler 
have shown. The inheritance is probably that of a weakly 
dominant unit-character. 

Flexed tail (/) (kinky?) 

In all cases the tail is permanently rigid over a varying portion of its 
length, this stiffness being particularly conspicuous proximally. The 
rigidity may be accompanied by permanent V-shaped, U-shaped, spiral, 
etc., flexures. ... — Hunt and Permar, 1928. 

In some of the short-ear stocks bearing wavy tail a com- 
plete right-angle flexure is found (11, 78, I4.8, 44)- The joint 
is solidified by the fusion of vertebrae and the presence of an 
osteosis. Sometimes two or three of these flexures may be 
present in the same tail. This character may or may not 
be that reported by Plate, 1910. Such heritable flexures are 
found in long-ear stocks, but the identity or relationship of 
these characters is uncertain. The inheritance of flexures has 
been described by Hunt and Permar as a recessive which 
sometimes fails to come to recognizable expression. 

Danforth, {27), in speaking of kinky tail (see Fig. 34) says: 

In these the caudal intestine instead of completely degenerating after 
the sixteenth day as in normal individuals, persists until birth as small 
remnants which become cystic or granular. Above these cysts the develop- 
ing cartilages are thrown out of alignment and finally ankylose with each 
other, forming permanent kinks. 

Posterior reduplication 

By selection there has been developed a strain of mice which give a 
high percentage of young showing varying degrees of posterior reduplica- 
tion. — Danforth, 1930. 

This character (see Fig. 32) ranges from Polydactyly to 
completely formed additional posterior parts including legs, 
genitalia, and alimentary tract {27). The inheritance of 
the character is recessive and approximates the behavior 


expected of a unit-character, with a deficiency probably 
due to its low viability. 

Waltzing (v) 

In 80 b.c. "in the ninth moon, a yellow mouse was found dancing with 
its tail in its mouth in the gateway of the palace of the Kingdom of Yen 
[now the province of Chili]. The animal danced incessantly. The king 
asked the queen to feed it with wine and meat but this did not interfere 
with the performance. The mouse died during the night. — Annals of the 
Han Dynasty (translation by Quentin Pan). 

Waltzing mice (see Fig. 22) are unable to orient themselves 
upon a horizontal plane, and this results in a rapid and erratic 
turning or whirling. Waltzers make many turning, twist- 
ing, and jerking head movements {183). Waltzers are totally 
deaf {182). Different investigators disagree as to whether 
or not the semicircular canals are morphologically normal. 
Waltzers are very delicate, poor mothers, and quite suscep- 
tible to cold. The most common inbred strain of Japanese 
waltzers in America is non-agouti black and bears a high 
grade of recessive piebald, with the anatomical characteristics 
of Mus bactrianus (wagneri). Waltzing is inherited as a sim- 
ple recessive (25, 31, 32, 61, 76). 

Gates (63) bred a waltzer containing but one dose of 
waltzing, which presumably through a faulty cell division 
had lost that portion of the normal chromosome which 
contains the allelomorph of waltzing. Painter's histological 
investigation (142) confirmed this conclusion. 

Shaker (sh) 

The mutation shows itself principally in the form of nervous head 
movements. — Lord and Gates, 1928. 

The shaker (126) makes choreic head movements similar 
to those of the waltzer but lacks the circling movements. 
The character is recessive and linked with albinism and pink- 
eyed dilution. 

Rodless retina (r) 

Microscopic sections of these eyes showed the total absence of visual 
cells (rods). — Keeler, 1924. 


This retinal defect (see Fig. 36) is characterized by com- 
plete absence of rod and external molecular layers and a 
great reduction of the cell number in the external nuclear 
layer (86, 90). The condition is readily detected by examin- 
ing histological sections of the retina. The iris of the rod- 
less eye contracts (88, 89) upon exposure to light and the 
blindness of rodless mice may be detected only by precise 
and carefully conducted animal-behavior tests (87). Rod- 
less eyes secrete no visual purple (90) and produce no elec- 
tric action current responses (99) to stimulation by light. 
The recessive gene producing rodless retina is linked (98) 
with that producing silver. 

Dwarf (dtv) 

In the case of dwarf mice, mature individuals are only about one-fourth 
the weight of their normal brothers and sisters, scarcely bigger, in fact, 
than the ordinary mouse 16 or 17 days old. — Snell, 1929. 

These creatures (see Fig. 33) are probably pituitary- 
deficient dwarfs. 1 The character is determined by a reces- 
sive gene (160). 

Hyperglycemia (hy) 

The findings obtained with the second generation confirmed the con- 
clusion that hyperglycemia, like albinism, is a recessive character. — 
Cammidge and Howard, 1926. 

It has been definitely established (13) that a certain strain 
of mice bear a gene, which in the homozygous state raises 
the fasting blood-sugar proportions. Whereas normal mice 
have a blood-sugar content of 74-84 mg. per 100 cc. of blood, 
the mutant strain bears from 113-124 mg. per 100 cc. The 
factor concerned is not linked with albinism but further 
genetic analysis is wanting. 

Other characters. Numerous other mouse variations are 
known to breeders of mice. Some of these characters are 

1 A paper by P. E Smith and E. C. MacDowell in the Anatomical Record, vol. 
46, p. 249, published too late for inclusion in the bibliography, confirms that the 
dwarfed condition is due to anterior pituitary deficiency and may be corrected by 
injection of rat pituitary. 


erratic in their appearance and may depend upon several 
factors, genetic or environmental. Hagedoorn reported a 
genetic modifier of blue dilution (72). Heterozygosity for 
brown, pink-eye dilution, or albinism also has a tendency 
to lighten the coat. Little and Tyzzer {122) believe that 
susceptibility to development of a certain sarcoma found in 
the Japanese waltzing mouse is dependent upon three or four 
independently inherited Mendelizing factors. The evidence 
is derived from a cross between Japanese waltzing mice and 
ordinary fancy mice in which the first generation was suscep- 
tible but in the second generation there appeared a certain 
percentage of mice of non-susceptible constitution. Other 
tumor susceptibilities (see Fig. 31) have been demonstrated 
to be hereditary {121, J>3, 12k, 165). 

Several times in recent years a black-silver strain of mice 
has thrown occasional individuals bearing a symmetrical 
lacing pattern of white hairs. Its genetics is complex. Con- 
genital cataract and stationary pupils have been found to 
run in certain mouse families. A dilution effective in com- 
bination with pink-eyed black was found several years ago, 
but has not been completely analyzed. Recently a separa- 
tion of the metopic suture, parted frontals (see Fig. 35), was 
reported (97) as a dominant unit-character. A twisted con- 
dition of the nasal bones behaving irregularly in heredity 
was also found (96). 

Another strain of mice produces individuals lacking kid- 
neys (6), or having lesions of the eyes, head, or feet (120). 
These are probably related, or expressions of the same 
process determined by the same genetic factors. 

Nomenclature. The compound breeds of the house mouse 
are merely combinations of the simple characters already 
described. It is customary in scientific circles to designate 
the breeds by analytical names, but the fancier and layman 
often employ other terms, usually descriptive. In the follow- 
ing list are given the more common fancier's terms and their 
analytical equivalents followed by their genetic formulae 
in terms of mutated genes. 



Fancier's Term 

Scientific Term 

Genetic Formula 


Gray agouti, wild 

No mutations 



a a 


Brown agouti 



Non-agouti brown 

aa bb 

Fawn with pink eyes 

Pink-eye agouti 


Clear fawn with pink eyes 

Pink-eye brown agouti 

bb pp 

Lilac, blue-lilac 

Pink-eye non-agouti 

pp a a 

Champagne, cafe au lait 

Pink-eye non-agouti brown 

pp a a bb 

Blue, maltese 

Dilute non-agouti 

dd a a 


Dilute non-agouti brown 

dd aa bb 


Pink-eye dilute non-agouti 

pp dd aa 


Pink-eye dilute brown non- 

pp dd bb aa 


Yellow, lethal yellow 

A Y A 

Sooty, sable 

Sooty, sable (one of the ex- 
pressions of lethal yellow 
often carrying non-agouti) 

A Y a 

Cream, light yellow 

Dilute yellow 

dd A Y A 

Pied, piebald, Dutch spotted 

Recessive spotting 



Dominant spotting 


Black-eyed white 

Black-eyed white 

Ww ss 


Yellow-belly non-agouti 

aa 1 , a t a t 


Y T ellow-belly non-agouti di- 

dd aa 1 , dd a l a f 

Chocolate-and-buff or 

Yellow-belly non-agouti 

bb aa 1 , bb a l a l 



Yellow-belly gray 

Yellow-belly agouti 

A w A,A W a or .4 a' 


May refer to several com- 
binations, the most com- 
mon of which is probably 
yellow-belly non-agouti 

a l a l ss, a 1 a ss 


Recessive hairless 

hr hr 

Japanese waltzer 

Usually non-agouti piebald 

aa ss vv 

Short ears 

Short ears 

se se 

Some of the more common fancy varieties of mice are 
figured in color by Schuster (157) and Little (110). 


Indeed they (white mice) always bring forth white ones. 

— Pallas, 1766. 

For many years it has been known that when white mice 
are crossed with pure-bred grays, the immediate offspring 
will be gray and in the second generation albinos will reap- 
pear (36). The true nature of this transmission, however, was 
first clearly understood in 1900, when Mendel's Law was 
rediscovered. 1 

Work since that time has shown that the majority of 
house-mouse characters are inherited in the same simple 
fashion as albinism. Most of the fancy characters are reces- 
sive like albinism, which, when crossed with the wild gray 
type, produces all grays in the first generation and one 
recessive out of four in the second generation. 

The mechanics of such inheritance is clearly known. The 
physical determiners underlying these hereditary variations 
are located in the minute rod-shaped bodies (chromosomes) 
within the nucleus of each cell of the body- In all mouse 
body-cells there are twenty kinds of these chromosomes 
(see Fig. 9) and two of each kind, one of each pair having 
been received from the father and one from the mother. 
These chromosomes are reduced from the diploid (double) 
number to the haploid (single) number at the formation of 
the gametes (eggs and sperm), so that each gamete contains 
only one of each pair. When two gametes unite (an egg with 

1 litis, 1924 (.SO), p. 68, says: "We have already heard from Fr. Hornish and 
Inspector Nowotny that Mendel raised mice in one of his two rooms, and not only 
white ones but also grays, and crossed them with each other. It is very possible that 
through these more dramatic researches the revelation of dominance and segrega- 
tion appeared to him for the first time. Indeed, he mentions nothing about it. 
This is not to be wondered at because industry in natural science at once made an 
ecclesiast suspicious in the eyes of many clerical zealots, to whom the undertaking 
of animal breeding appeared highly immoral." 



a sperm) to form an embryo, the double number is again 

A pure-bred gray-coated mouse receives a determiner for 
pigment development (C) from its father and one from its 
mother. When its gametes are formed, each will contain 
a single pigment determiner. Hence, in matings with 
another pure-bred gray, a pair of pigment determiners will 
enter into each embryo and nothing but pure-bred pigmented 
young will be produced. 

In like fashion an albino receives one determiner for 
albinism (c) from its father and another from its mother, and 

Fig. 6. Diagram illustrating the inheritance of a simple, recessive, mendelizing unit-character 
such as albinism. Black wafer = C gene, white wafer = c gene. 

produces gametes each bearing a determiner for albinism. 
Thus, when two albinos are mated together their offspring 
will receive a determiner for albinism from each parent and 
hence all are albinos. 

If an albino is mated with a pure-bred gray, the albino 
will contribute an albino determiner (c) and the gray will 
contribute a pigment determiner (C) (see Fig. 6). It so hap- 
pens that the pigment determiner in this case completely 
dominates over the albino determiner and the cross-bred 
offspring are pigmented, giving no evidence that they carry 
an albino determiner. A cross-bred gray carrying albinism 
produces two kinds of germ cells in equal numbers, half 


containing the pigment determiner and half containing the 
albino determiner. Now if two cross-bred grays, each carry- 
ing an albino determiner, be mated, the two kinds of germ 
cells (C and c) will come together purely at random and the 
offspring will appear in the proportions of 1 pure-bred pig- 
mented: 2 cross-bred pigmented: 1 pure-bred albino. The 
visible classes will be 3 pigmented (containing C) : 1 albino. 

Gametes of father 
C c 

of mother 




Combinations among 
the offspring 

An actual experiment of this sort (combined data from Little 
{110) and Keeler {87)), produced 2994 pigmented young and 
1029 albinos or a ratio of 2.91 to 1, the theoretical expecta- 
tion being 3017.75 pigmented: 1005.75 albinos. 

Some of the mouse genes have more than two alternative 
forms or different chemical states. The color gene {€) has 
at least four such allelomorphs in mice. These are (1) normal 
pigmentation (C), (2) chinchilla {c ch ), (3) extreme dilution 
or Himalayan albinism {c H ), (4) complete albinism (c). 

Each member of the series dominates in crosses over those 
which follow it in the same series. That is, when an animal 
bears heredity for any two of these characters, the one 
appearing first in the list will be expressed in the coat. For 
example, if a chinchilla be crossed with an albino, the first 
generation will be chinchillas and the second generation will 
contain an average of 3 chinchillas to 1 albino. No more 
than two members of such a factor series may exist in a 
single individual (one in each of the two chomosomes making 
up a particular pair). 

If a normally pigmented animal carrying a chinchilla 
factor be crossed with an extreme-dilute animal carrying 
albinism, the first generation will consist of equal numbers of 
normally pigmented animals and of chinchillas. Half each of 



the normals and of the chinchillas will carry extreme dilution 
and half will carry albinism. 



C c H 
normally pigmented 

c ch Q H 

C c 
normally pigmented 

c ch c 


If these hybrid-generation animals be mated at random 
four kinds of germ cells will be formed in equal numbers and 
will unite also at random, producing on the average 7 normals, 
5 chinchillas, 3 extreme dilutes, and 1 albino. Of the 7 nor- 
mals, 1 will be pure bred, 2 will carry chinchilla, 2 will carry 
extreme dilution, and 2 will carry albinism. Of the 5 chin- 
chillas, 1 will be pure bred, 2 will carry extreme dilution, and 
2 will carry albinism. Of the 3 extreme dilutes, 1 will be 
pure bred, and 2 will carry albinism. The 1 albino will be 
pure bred. See diagram. 

C c ch c H c 



CC Cc ch 
normal pigment normal pigment 

Cc H Cc 
normal pigment normal pigment 


c ch c c ch c ch 
normal pigment chinchilla 

c ch C H 

c ch c 


c H C 
normal pigment 

C H c ch 

C H C H C H c 
extreme dilute extreme dilute 


normal pigment 

c c ch 

cP c cc 
extreme dilute albino 

Suppose we have a mouse bearing two independent unit- 
characteristics (borne in different chromosomes), for ex- 
ample, rodless and albinism. The determiner for the albino 
character lies in one chromosome pair and that for rodless 
in an entirely different pair. The genetic formula of this 



mouse with respect to the two characters named will be 
rrcc, while that for a normal-eyed pigmented mouse will be 
RRCC. The gene for rodless is represented by r, the gene 
for normal eye by R; the gene for albinism by c, the gene for 
colored coat by C. 

If we cross a rodless albino with a normal-eyed pigmented 
mouse all the immediate offspring will be Rr Cc. The gene 
for normal eye dominates over the gene for rodless and thus 
the offspring are normal eyed. The pigment-forming gene 
dominates over the albino gene and thus the offspring are 
pigmented. Yet they carry (recessive) heredity for both rod- 
less and albinism. 

These normal-appearing offspring, carrying as recessive 
genes albinism and rodless, form four kinds of gametes in 
equal numbers, RC, Re, rC, and re. If two such double 
heterozygotes be mated together they will produce offspring 
in the proportion of 9 normal-eyed pigmented, 3 normal- 
eyed albinos, 3 rodless pigmented, and 1 rodless albino. 
The checkerboard below shows how these combinations 
could arise through the random union (48) of these four types 
of gametes. 


Re rC 


























An experiment (87) of this kind yielded 165 normal-eyed 
pigmented, 64 normal-eyed albinos, 51 rodless pigmented, 
and 29 rodless albinos, or a ratio of 8.5 : 3.3: 2.6: 1.5. 

















If the double heterozygotes be mated back to the parent 
pure for the two recessive characteristics, four classes of 
offspring will be expected in equal numbers, namely, normal- 
eyed pigmented, normal-eyed albino, rodless pigmented, rod- 
less albino. This test (87) yielded of those classes respectively 
66: 54: 68: 51 or ratios of 1.1: .99: 1.1: .93. 

Gametes of double heterozygotes 
RC Re rC 

Gametes of re 





















The gene for two or more unit characters may lie at differ- 
ent points or "loci" upon the same chromosome, a relation- 
ship known as linkage. Linkage is measured by the amount 
of crossing-over, or recombination. Ordinarily in the forma- 
tion of gametes the two chromosomes of an homologous 
pair, which have lain side by side in immature germ cells, 
separate at maturation and one passes to each daughter cell. 
Each functional gamete (egg or sperm) thus contains one 
representative (not two) of each gene. The chances are even 
that it will be the representative derived from the father or 
that derived from the mother. 

Occasionally chromosomes break (see Fig. 7) transversely 
while lying elongated in pairs previous to the reduction 
division. When the break is repaired the broken ends may 
become united differently from before. The chromosomes 
which originally bore RC and re respectively may after a 



"crossing-over", bear Re and rC. The percentage of cases 
(times in a hundred) in which crossing-over occurs is called 
the crossover percentage. The nearer together two genes 
lie in the same chromosome, the fewer chances there are for 
breaks to occur between them resulting in recombination 
or crossing-over. Hence the percentage of crossing-over is 
taken as a measure of the nearness together of genes or of 
the linkage strength (16) between them. 

In cases where linkage is involved, new combinations 
(recombinations) of two characters may be difficult to obtain. 

Fig. 7. Diagram of a pair of chromosomes showing the linkage relationship of two 
pairs of genes before, during, and after a crossing-over. 

Suppose we select a mouse bearing two genes located in the 
same chromosome, for example rodless and silver (rrss). 
Should we cross this rodless-silver to a normal-eyed unsil- 
vered mouse (RSRS) the offspring will be rsRS and will be 
normal-eyed unsilvered. When these animals form gametes, 
r and s being in the same physical chromosome will behave 
ordinarily as units as also will R and S for a like reason. 
Hence there will be ordinarily two, and only two, kinds of 
gametes, namely, rs and RS. If two such animals are mated 
together they will produce ordinarily but two types of 
offspring: normal-eyed unsilvered (RSRS or RSrs) and rod- 
less silver (rsrs). But when there is a crossover, two other 
types of gametes are produced (rS and Rs), making possible 
the occurrence of occasional rodless unsilvered animals (rSrS 
or rsrS) and normal-eyed silvers (RsRs and rsRs). We may 
detect the percentage of crossovers between r and s by 
crossing an rsRS animal to an rsrs (see Fig. 8). 



Gametes of rsRS parent 
RS Rs rS 























Crossover classes 

If the characters silver and rodless were not linked, we 
should expect equal numbers of all four classes. If they are 
linked, we expect the second and third classes to be small in 
comparison with the first and fourth. The second and third 

Fig. 8. Diagram of a back-cross illustrating the linkage between rodless retina (r) 
and silver pelade {x). Animals with unshaded eyes are rodless. 

classes are produced only from gametes in the genesis of 
which a crossover has occurred between silver and rodless. 
A back-cross of the kind described (98) gave 9 unsilvered 
normals, 1 unsilvered rodless, 1 silver normal, and 5 silver 
rodless. More extensive breeding tests indicate that a cross- 
over takes place between silver and rodless at the formation 
of about 12 per cent of the gametes. 

If three determiners are located at different points in the 
same chromosome, as are albinism (172), shaker (126), and 
pink-eye dilution (172), we may determine their relative posi- 
tions by the percentage of crossovers obtained in breeding ex- 
periments. If a crossover occurs between albinism and shaker 
3 times out of a hundred and between shaker and pink-eye 
dilution 15 times out of a hundred and between albinism and 



pink-eye dilution 18 times out of a hundred, then the three 
genes must lie in the order: albino, shaker, and pink-eye 
dilution. The indicated distance between albino and shaker 
is about one-sixth that between albino and pink-eye dilution. 
In Fig. 9 a number of determiners (genes) for mouse 
unit-characters are represented as located upon diagram- 
matic chromosomes of the mouse, to illustrate the genetic 









agouti I 

white - 

u u 



Fig. 9. Diagram of chromosomes of the house mouse (22) with genes for unit 
characters distributed arbitrarily among them to show genetic independence or 
linkage of the characters. 

independence or linkage found experimentally to exist be- 
tween these determiners. 

Besides the two linkage systems already mentioned {172), 
linkage has been shown to exist between recessive spotting 
and recessive hairless (161), and between short ears and 
dilution (64-, 159). It is possible, as already suggested, that 
agouti and white-belly may also be linked characters, rather 
than allelomorphs. 

According to present data, known hereditary characters 
have their determining genes located in 9 of the 20 pairs of 
chromosomes of the house mouse. It is reasonable to sup- 
pose that some day characters will be found in the remaining 
11 pairs as well as other genes within those chromosomes 
already bearing determiners for fancy variations. 



The characters non-agouti, pink-eye dilution, blue dilution, 
brown, recessive spotting, waltzer, rodless, shaker, and the 
albino series of allelomorphs seem to be recessive unit- 
characters and to behave according to the rule for simple 
Mendelian inheritance. Varieties characterized by these all 
breed true because they are homozygous. Yellow-belly 
behaves as a dominant unit-character and breeds true only 
when homozygous. Yellow and dominant spotting do not 
breed true but produce only 50 per cent of offspring possess- 
ing the characters in question, because they are dominant 
lethals and so cannot be made homozygous. 

Normal Overlaps. Silver behaves as a recessive unit- 
character but seemingly does not always breed true because 
some silvered animals are with difficulty recognized as such. 
Silvered mice throw a certain percentage of animals of such a 
low grade of silver that they may not be distinguished read- 
ily from non-silvered animals, although they are genetically 
silvers. Such animals which are genetically of a mutant type 
but somatically are capable of classification as of the normal 
type are known as normal overlaps. 

Dominant Lethals. Two of the unit-characters of mice, 
namely yellow and dominant spotting, are dominant lethals. 
By this we mean that animals bearing one factor for the 
character exhibit the character, but that animals bearing 
two factors for the character are non-viable. Thus, all yellow 
mice carry a non-yellow factor as a recessive. When two 
yellows are mated together they produce on the average one 
homozygous yellow which dies, two yellows heterozygous 
like themselves, and one non-yellow animal, as was first 
pointed out by Cuenot (££). 

The same is true for dominant spotting. Two dominant- 
spotted animals produce on an average one homozygous 




dominant-spotted animal which dies of anaemia (115), two 
dominant spotted (heterozygotes), and one non-dominant- 
spotted animal. Strains of dominant spotting exist which 
throw self animals as recessives. (See Fig. 21.) These are 
the non-dominant-spotted class. Other strains, known as 
black-eyed whites, are homozygous for recessive spotting 
and consequently produce piebald animals instead of selfs, 
as the non-dominant-spotted class. 

Hereditary Sterility. Dominant hairless (naked) individ- 
uals which are heterozygous are often produced, but are 
usually sterile. In one case a homozygous naked female pro- 
duced young but had no milk to nurse them. Possibly the 
mutation which affected the hair follicles also disturbed the 





Homozygous naked 


Heterozygous naked 


Heterozygous naked 



processes of mammary development or secretion. Dominant 
hairless stocks are maintained by mating heterozygous indi- 
viduals with each other or by mating heterozygotes to 
normals. In both types of cross one half the progeny are 
wasters, that is, are not of the desired type, heterozygous 




Heterozygous naked 

Heterozygous naked 



Recessive hairless usually produces sterile females and 
fertile males and hence the strain is readily preserved by 


mating hairless males to heterozygous females. The resulting 
hairless males are crossed to their heterozygous sisters, and 
so on. 

A certain amount of sterility is encountered in short-eared 
mice. This may or may not be associated in some fashion with 
the short-ear character, but is complicated in its inheritance. 

The sterility often found among waltzers is usually due to 
the fact that these animals are very delicate and require the 
best of care in a very special environment. Under ideal con- 
ditions they breed well. They are, however, poor mothers 
and their young should when possible be given to a foster 
mother of some non-waltzing variety to nurse. 



Mating Habits. The oestrous cycle (2) of the mouse lasts 
from three to four days. In only a few hours within this 
cycle will the female permit the amours of a male; at other 
times she avoids his attentions by slipping quietly into the 
nest or by climbing up the side of the cage. 

During the receptive period her reactions to courtship 
change completely. When her suitor approaches, she often 
rears upon her haunches, throws her paws up in a defensive 
attitude, closes her eyes, and gives a characteristic short 
squeak. The male nervously licks her face, sniffs at her 
genitals, and attempts to mount. The female often runs 
away into the nest or up the side of her cage only to return to 
the former trysting place. There are numerous unsuccessful 
attempts at copulation before the completed act takes place. 

The secretions of the male form a soft vaginal plug which 
quickly hardens, cementing the vagina shut and thus pre- 
venting escape of the sperm until after fertilization (125) of 
the eggs is accomplished. The vaginal plug is usually lost 
within twenty-four hours. 

Uterine Development (129, 81). The sperm ascend the geni- 
tal tracts within a few hours after copulation and fertilize 
the eggs as they are shed from the follicles before they enter 
the uterus. The fertilized eggs descend into the uterus where 
they implant about the fifth day. At this time development 
has progressed to about the primitive streak stage. About 
the eighth day, at a stage corresponding to that of the three- 
day chick, all the foetal organs are laid down. The cerebral 
ganglia are growing out, and the limb buds are forming, 
while the heart has long been pumping blood through the 
foetal arches. The retina has invaginated and the lens is 



The young are born usually between the nineteenth and 
twenty-first days after mating {103, 29). In Japan wild 
house mice and those of the fancy are often termed "twenty- 
one-day mice" on account of the length of the gestation 

Birth. Just prior to parturition the female may be ob- 
served to become restless. During this period she may con- 
struct an elaborate nest by winding strands of nesting tissue 
to form a ball, tucking in loose ends here and there. 

At the birth of young, the reactions of the mother seem 
to vary greatly with such factors as her natural temperament, 
health, company, and the degree of seclusion provided by her 
cage. Some mice retire to the nest for parturition while 
others continue their routine reactions about the cage. 

Reaction to the new-born young may vary from almost 
completely ignoring them where they happen to drop out- 
side the nest, to birth within the nest followed by careful 
washing and cuddling. 

Four or five minutes of uterine contraction may be re- 
quired to give birth to a mouseling, followed within a minute 
by the expulsion of the placenta and perhaps portions of 
the fcetal membranes. The female chews and occasionally 
devours the placenta. 

The new-born mouse is a naked little creature about two 
and a half centimeters long with well proportioned head, 
body, and feet. It is unpigmented save for the dark ring of 
the iris, which is visible through the translucent eyelids. The 
ears are folded forward and sealed. 

The new-born mouse left to itself may remain motionless 
for as long as a minute before it gives its first tiny gasp, 
followed by another and another. These gasps are con- 
tinued at irregular intervals, often accompanied by violent 
contractions of the body and especially the breathing 
muscles. Within five minutes the mouseling is breathing 
regularly and lying quietly except perhaps for an occasional 
twitch of a leg or a sucking movement of the mouth. 

Nursing takes place as soon as the mother huddles over 


the young mouse subsequent to its metamorphosis into an 
air-breathing creature, at times perhaps even before it is dry. 
The first period of nursing may continue as long as fifteen 
hours, if the mother is not interrupted. 

Under poor conditions some mice will not nurse their 
young. In case a mouse refuses to nurse young of great 
value to the investigator, they should be fostered. If young 
mice have nursed it is evident, because milk contained in the 
stomach shows through the left side as a cream-colored 

Growth. As with most helpless infants, during the first 
period of their lives, the reactions of young mice are con- 
fined chiefly to nursing and sleeping, while their bodies grow 
rapidly in size and differentiation progresses in the more 
retarded organs such as those of special sense. The rods of 
the retina, for example, are developed after the fifth day. 

The pigmented hair within the skin is faintly visible upon 
the second or third day and is well developed by the eighth 
day, when the skin becomes scaly, probably due to a shedding 
of the external surface as dandruff. By this time the little 
mice move blindly about the nest and even venture into the 
world outside, only to be hustled back in the mouth of their 
mother, who continually washes, feeds, and looks after them. 

By the thirteenth day the truancy of these little balls of 
fur, still unsteady upon their feet, becomes too great for their 
attentive mother, who still repeatedly carries them back to 
the nest. They occasionally escape to a sequestered corner 
of the cage, where they test things with their paws, noses, 
tongues, and vibrissa?. They sit and wash their faces, nibble 
morsels of food, and blink at the light with their newly 
opened eyes. 

In the childhood of the mouse there has been observed no 
period of play such as young rats enjoy when they mouth, 
kick, tussle, clutch, and roll with each other. However, be- 
tween the fifteenth and twenty -fifth days young mice unused 
to handling are more restless and more active than usual, 
squeaking, scampering, leaping, and seeking to hide them- 


selves at the slightest noise or motion. The degree of this 
activity also has a genetic basis, because members of certain 
strains are much more active than members of other strains. 

By the thirteenth day the young mouse is almost an adult 
save in size and sex differentiation. Sex maturity is usually 
reached between the second and third months, although 
mice increase a small amount in size during the several 
succeeding months. 

Fostering. Fostering is often desirable where the female 
is of a feeble strain, is a poor mother, or is one from which 
a maximum number of offspring is desired. 

In fostering it is a good practice to employ as nurses 
females which are first-generation offspring of a cross between 
two inbred varieties which show great vigor, although any 
docile vigorous female will do. A number of these prospective 
nurse mothers are mated simultaneously with those whose 
young are to be fostered so that nurses will be available 
when the desirable young are born. 

Some foster mothers object to an exchange of young, espe- 
cially if the mice to be fostered are younger than her own 
litter. The foster litter and the foster mother's own litter 
are shaken together gently by some investigators in order 
that the foster mice may obtain the odor of the foster moth- 
er's nest and be more acceptable. The mice to be fostered 
are picked out and given to the foster mother, while the 
foster mother's own litter are killed. 

If a female mouse has not nursed much, she will usually 
breed within twenty-four hours after parturition. 

Killing. Some investigators prefer to drop all discard mice 
into a covered jar containing a piece of cotton saturated with 
ether, although simple mechanical methods of terminating 
their existence are both adequate and painless. 

Parasites. Even with the best of care a mouse colony may 
occasionally be infested with fleas, mites, or lice to the ex- 
tent that they prevent breeding, although these parasites 
seldom prove fatal. Pyrethrum powder or pulverized tobacco 
dusted upon the animals from time to time are good preven- 


tive measures. However, should the infestation persist, the 
mice may be individually caught by the tail, dipped in a 
warm, very dilute solution of stock-dip, and allowed to dry 
in a warm place. If metal cages are employed, the nest and 
food may be removed and the whole cage dipped, mice and 
all. We have found Parke Davis and Company's Kreso-Dip 
satisfactory for this purpose. 

Mice are occasionally attacked by a white fungus which 
grows upon their ears. It is difficult to eradicate and hence 
if the affected animal may be spared it should be killed. If 
the fungus affects only the tip of an ear when discovered, it 
may frequently be eliminated by clipping off the ear below 
the affected region. Adult mouse ears seldom bleed when 
snapped off, but if they should, a little sodium subsulphate 
powder will quickly stop the flow. 

Mice sometimes harbor a tapeworm for which the cat 
serves as definitive host. Accordingly, it is advisable to 
exclude all cats from the mouse room. There are other more 
obvious reasons why cats are undesirable tenants of a 

Diseases. Some strains of fancy mice are affected with 
hereditary tumors. Those involving the mamma? of the 
female are the most common. If a tumorous female is very 
valuable, occasionally a single litter of mice may be pro- 
cured from her after surgical removal of the growth. As a 
general practice animals bearing tumors should be discarded 
as the results seldom justify the efforts to save them. 

Mice are susceptible to a number of non-specific organism- 
borne diseases {171, 133), such as surra {Trypanosoma evansi), 
apoplectic septicemia, fakosis {Micrococcus caprinus), fowl 
cholera {Bacterium cholerce gallinarum), trichosis {Trichinella 
spiralis), the disease caused by Bacillus piliformis, sarcospiri- 
diosis, botryomycosis, and coccidiosis. These diseases will 
seldom be encountered in a well-kept mouse colony, and 
hence a mere mention of them will suffice here. 

By far the worst disease among laboratory mice is para- 
typhoid {175) (often known as diarrhoea) caused by Bacillus 


typhi murium. If one can possibly spare the mice, it is 
advisable to kill off each day all sick animals and sterilize 
the cages at once. Some adult mice are not killed outright 
by the disease, but they remain carriers of the condition and 
may do much damage by spreading it. This disease is 
usually fatal to animals between fifteen and twenty-five days 
of age. A few recover, and if these are females they may 
be normal and reproduce, but if they are males they will 
usually be sterile, due to the fact that the poisonous, diar- 
rhoeic feces "scald" the scrotal region, causing the forma- 
tion of scar tissue. The stiff scar tissue prevents the descent 
of the testicles necessary for fertility. Thus it is advisable 
in every case to kill all young affected males. If valuable 
young females are preserved, they should not be kept in the 
mouse room. 

By the enumeration of the above pathological conditions 
and insistence upon drastic measures to eliminate disease 
from the mouse colony, it is not intended to give the im- 
pression that mice are weak and susceptible to all sorts of 
sickness, making them difficult to raise in quantity. There is 
no limit to the number of healthy mice which may be main- 
tained in a well-tended murarium. Five thousand is not an 
unusual size of colony for an experimental laboratory. 

Breeding Cages. There are a number of practicable de- 
signs for successful breeding cages, but the most satisfactory 
are those which provide adequately for certain requirements 
peculiar to the mouse. The design of the cage should take 
into consideration size convenient for handling, space oc- 
cupied, dry feed, closed water bottles, changeable nesting 
material, absorption of urine, and ease of sterilization. A 
mouse cage meeting these requirements has been developed 
at the Bussey Institution (see Fig. 10). 

The cage is made of galvanized-iron wire netting of 
^ inch mesh. The dimensions of the cage are 12 inches long, 
7 inches wide, and 7 inches high. It rests upon half an inch 
of sawdust in a galvanized-iron pan 15 inches long, 13| 
inches wide, and 3| inches deep. Two cages nest side by 


side in each pan. These pans with the contained cages are 
kept upon racks of dimensions to accommodate them. 

The cage proper has a sloping front provided with a gal- 
vanized-iron door which is held shut by gravity and the 
weight of the water bottle. The iron door is convenient as a 
memorandum space for numbers or notes not recorded in the 
official register. 

A dog biscuit is wired to the rear wall. The cage is pro- 
vided with a handful of shredded tissue paper for nest ma- 

Fig. 10. Wire mouse cage used at the Bussey Institution. 

terial and with a feed dish. A numbered metal tag for cage 
identification is wired to the sloping front, or the number 
may be painted on the iron door. 

Once a week the cages should be cleaned with a stiff brush, 
provided with fresh nesting tissue, fresh sawdust, and clean 
food dishes, and about once a month they should be washed. 

Sanitary Precautions. Because mice are subject to several 
infectious diseases, cages should be of a material which may 
be readily sterilized. If cages are made of metal they may 
be sterilized by immersion in boiling soapy water, brushing, 
and then dipping in a solution of "Kreso" or other disinfec- 
tant. Cages should be disinfected at least once a month 
in absence of disease and oftener if the colony is infected. 
Cages in which diseased animals have lived should be dis- 
infected immediately. 

Animals dying from no matter what cause should be 
removed from cages at once, because the carcasses are often 
partially eaten by other mice and disease may thus be spread. 


Water. Water standing in an open dish in a mouse cage 
becomes quickly contaminated with urine and feces and is 
unfit for drinking. The greatest difficulty, however, is that 
active mice will continually run through any open water 
dish, very frequently splashing water upon their fur. Often 
they become chilled and contract pneumonia, which usually 
proves fatal. 1 

For these reasons a closed drinking-water supply is best. 
A bottle fitted with a rubber cork pierced by a glass tube is 
most satisfactory. The glass tubing should be drawn to a 
nipple and the end smoothed in the flame, because water 
will drip through a large opening. This provides constantly 
a hanging drop which is licked by the mice when they 
desire it. 

Records. Every animal should be numbered and registered 
with regard to individual number, sex, description, known 
recessive characters, parents, date of birth, and often disposi- 
tion and date. 

A card index should be kept, bearing the numbers of the 
cages upon the guide cards and a card filled out for each 
animal with data on individual number, sex, purpose and 
date of mating, and cage number. This should be filed at 
the proper place within the index. 

When a female becomes pregnant, she should be given a 
separate cage, because, with other mice in the cage, some 
mothers become excited and kill their young. New-born 
mice are often killed by males or more often still by other 
females in the cage. Date of birth, record number of the 
father, and information concerning the young not yet reg- 
istered may be jotted down upon the card of the mother. 

Numbering. A satisfactory system of marking mice for 
identification purposes is to punch the ears with a chick 
punch. The following simple system of position marks is 
used almost exclusively by American geneticists. The first 

1 The practice of providing open water dishes may have occasioned the state- 
ment of Aristotle concerning the white mice of Pontis that if they drink water 
they will die. 


three numerals, 1, 2, and 3, are denoted by holes in the ear 
at top, side, and bottom respectively. The second three, 
4, 5, and 6, are represented by notches in the edge of the 
ear at top, side, and bottom. The next three numerals, 7, 8, 
and 9, are indicated by combinations of two notches, 7 being 
notches at both top and side, 8 being notches at both side and 
bottom, and 9 being notches at both top and bottom. 

With this system, employing the right ear for units and 
the left ear for tens, one may number animals from 1 to 99. 
One hundred has a hole in the center of each ear, but may be 
distinguished from 200, 500, 700 etc., by age, color, parents, 
cage number, and other records. By these data one may also 
identify mice whose individual record numbers bear the 
same last two digits. 

Temperature. Mice are quite sensitive to changes in 
temperature. A cold draft from a window may prove fatal 
to a high percentage of a colony within a single night. Even 
the opening and shutting of outside doors on cold winter 
days may chill the mice considerably. The optimum condi- 
tion is a room regulated between 70° and 80° F. both day 
and night. Animals suffering slightly from exposure may 
wheeze chronically, but appear in fair health and even breed. 
This condition is known to the fanciers as "singing" or 

Food. Food should be readily available to mice at all 
times. A mouse that spends more than twenty-four hours 
at one time without food is in grave danger of starving to 
death. If only a meager amount of food is present, life may 
be sustained, but the animals will not reproduce. A suitable 
ration for mice measured by weight consists of: 

240 parts rolled oats 
30 parts powdered skim milk 
8 parts cod-liver oil 
1 part salt 

A formula for rat feed, which is probably satisfactory also 
for mice, has been recently prepared by Maynard (131) 
consisting in parts by weight as follows. 


Linseed-oil meal 15 

Ground malted barley 10 

Wheat red-dog flour 22 

Dried skim milk 15 

Oat flour 15 

Yellow corn meal 20 

Steam bone meal ' 1 

Ground limestone 1 

Salt 1 

Total 100 

As mice are so dependent for health upon the constant 
availability of food, it is advisable to have present in their 
cages at all times a balanced-ration dog biscuit upon which 
they may gnaw if other food has been consumed between 
feeding hours. 

For mice to breed well, greens are desirable in the form 
of lettuce or clover once or twice a week. Caution must be 
observed in the feeding of lettuce that all tainted or rotten 
spots be removed, for mice will eat these along with the 
good portions and may be made sick by so doing. Hemp 
seed is advised from time to time. According to fanciers, 
animals suffering from lack of greens may become scurvied, 
but this condition disappears when they are properly fed. 

A Closed Feeding Can. Mice delight in digging in an open 
dish full of food. They waste great quantities of food by 
kicking it out of the dish, and contaminate with their feces 
that remaining in the dish. In order to eliminate these two 
undesirable features of the open feeding dish a closed feeding 
can has been recently devised and is in use at the Bussey 
Institution (see Fig. 11). This consists of a half-pound coffee 
can approximately 4 inches high and Sh inches in diameter. 
In the side of the can is cut a 1-inch square hole, the lower 
edge of which is h inch from the bottom of the can. The top 
and sides of the hole are faced with a strip of tin projecting 
\ inch into the can, to prevent the fall of food near the 
entrance. A wedge-shaped cage made of galvanized-wire 
cloth of \ inch mesh is soldered in place with the large open 
end of the wedge over the entrance and projecting about 


2^ inches into the can. The edge of the wedge rests upon the 
floor of the can and its sides extend outward on either side 
of the opening from which they are about \ inch distant 
laterally at the opening. 

The cage is small enough so the hind quarters of the adult 
mouse must remain outside the can while its hind feet rest on 
the edge of the entrance. These features prevent both fecal 

Fig. 11. Sectional view of feeding can. 

contamination and the kicking of food out of the can. The 
mesh of the screen-wire wedge is fine enough to prevent dry 
ration containing rolled oats from sifting into the feeding 
chamber. The mouse simply reaches through the mesh and 
pulls down what food he desires, eats it in place, and backs 
out of the feeder when satisfied. If it is desired to give the 
Maynard ration in the closed feeder, it should be mixed in 
equal parts with rolled oats, because the Maynard ration is 
finely ground and sifts through the screen wire. 





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