Ch. 9 — Advances in Reproductive Biology and Their Effects on Animal Improvement • 177 
surgical embryo reco\erv is pretei red for the 
cow anil the single o\ ulation of the horse. The 
appmach is especially important in dairy cattle, 
since it can he performed on the farm without 
interrupting milk production. 
.\o significant aihtinces i\in he ()i eilictt‘d foi’ 
the immediate future 
tAimtU) 1 It \Nsi lat 
Kmhryos I'an he remoxed from one animal 
and implanted into the o\ iiluct or utei'us of 
another. Ifoth surgical anil nonsurgical methods 
are currentlx in use. though success rates of the 
latter are much loxx er. 
The technologx can obtain offs|)ring from fe- 
males unable to support a pregnancy, increas- 
ing the number of offs[)ring from xaluahle fe- 
males and introducing nexx geni‘s into patho- 
gen-free heiils. Because more offspring can he 
obtained from the donor, undesirable recessixe 
traits can he ra[)iillx iletected. The technologx is 
also useil. along x\ ith short- oi' long-term stoi age 
of the emhrxos, as a means of trans[)orting 
germplasm rathei' than the xxhole animal. Cur- 
rent harriers to its further use are the costs in 
personnel and equipment, especially foi' surgi- 
cal prix'edures. anil the [)roxision of suitable 
recipients for a successful transfer. 
The use of embryo transfer should increase 
in the future, especially xxith animals of high 
xalue. Nonsurgical methods xxill increasingly 
replace surgical ones, especially for coxvs and 
horses. .A role for embryo transfer can also he 
predicted in progeny testing of females, obtain- 
ing txvins in beef coxvs, obtaining progeny from 
prepubertal females, and in combination xxith 
in X itro fertilization and a xariety of manipula- 
tixe treatments (production of identical txvins, 
selling or combining ox a from the same animal, 
genetic engineering). 
EMBRYO STORAGE 
The ability to store embryos increases the 
adx antages of embryo transfer procedures, loxv- 
ers the cost of transporting animal germplasm, 
and reduces the need to synchronize estrus in 
recipients. It xvill also be important in the study 
and control of genetic drift in animals. 
.Adequate culture sysliMiis I'.xist for shoi t-term 
storage ot embryos. I hex hax e hiu’n dex eloped 
hx trial-and-erior anil an* not optimallx’ di'fined 
lor farm specii's at presi'iit. N'ex crtheless, coxx' 
embryos haxe been stored for 'A days in the tieil 
ox iduct of a rabbit. 
I.ong-term storage, or freezing of embryos, 
exists, hut protocols nei'd to he impi'oxinl. As 
manx as txx o-thii’ds of the stored embryos liie 
xxith [jresent methods, lloxvever, for some uses 
embryo freezing is already pi’ofitahle. 
In the luture, tlu‘ di'xelopment of prei'ise em- 
hrxo cultui'e technolog^x' xxould help the dexel- 
opment of all ti'chnologies inxoixing the pro- 
longed manipulation of gameti!s and embryos 
outside the reproductix c tract. lA I'ntually, as 
freezing technology improxes, ni^arly all em- 
hi-yos taki'n from cattle in North .AnuM'ica xvill 
he stored, rather than transferred immediately. 
It appears that emhi’X'os successfully storeil xx ill 
surxixe foi- sexi’ial centuries and possibly foi- 
millenia. 
SE.\ SELECTION 
rhe ability to di’termine the se.x of the un- 
born, or of sperm at fertilization, xxill have nu- 
merous |)ractical and experimental applications. 
The most reliable method is karyotyping, by 
means of xx hich nearly txx'o-thii'ds of embryos 
can he sexed. Another method, xvhich tries to 
identify sex-specific pi’oilucts of certain genes, 
is under dexelopment. A reliable method for 
separating male-producing sperm from female- 
producing sperm has not been achieved, though 
sex eral patents are held on x arious tests of this 
type. 
Before any method has any practical effect on 
the production of farm animals it must become 
simple, fast, inexpensix e, reliable, and harmless 
to the embryo. The present state of the art is 
largely a consequence of research in male fertil- 
ity and in sperm survival after frozen storage. 
TWINNING 
Twins can be artificially induced by using 
either embryo transfer or hormonal treatments. 
The first approach is more effective. Selection 
among female sheep for natural twin produc- 
