Similarly when the fungicide Arasan (tetra- 
methylthuiramdisulfide), which is often used 
to protect seed, finds its way into the feed of 
laying chickens at 100 p.p.m., calcium metab- 
olism is affected and soft-shelled eggs are 
produced (Swanson et al., 1956), Nicarbazin 
(Weiss et al., 1960) and glycarbylamide (Polin 
et al., 1961) used in the control of coccidiosis 
in chickens have an adverse effect on fecundity, 
but none of these drugs has longlasting effects. 
Elder in 1964 screened two anticholesterole 
compounds (SC-12937 and SC-11952) to see 
whether they might interfere with yolk forma- 
tion in the rock dove (Columbia livia). His re- 
sults were spectacular. When administered for 
15 days at 0.1 percent in the diet, there was 
complete inhibition of ovulation for 3 months 
and very gradual recovery over a period of 
several months. We have confirmed this 
potency in quail. These hypocholesterolemic 
agents seem to be the mose potent female 
gametocides available to date for the control 
of overpopulations of birds. They are new and 
more research is needed on their possible 
side effects, not to mention on the economics 
of their industrial production and techniques of 
selective administration. 
Most diseases affect wildlife populations 
through biotic decimation. Newcastle disease 
virus (NDV), however, acts as a biogenetic 
decimator--temporary inhibition of egg laying 
occurs in the affected birds. The disease is 
almost universal among chickens, and most 
flocks are vaccinated against it before the 
chickens reach sexual maturity. Many species 
of wild birds, especially those in contact with 
poultry, are susceptible to Newcastle disease 
(Brandley, 1959) as shown by serum titer. It 
can be hypothesized that an isolated breeding 
colony of birds, clean and not yet immune to 
some strain of NDV, could be artificially in- 
fected just prior to the egg-laying season and 
rendered sterile for that season. Most other 
virological approaches to population control 
would seem to be fraught with hazards of great 
seriousness, 

Embryocides 
Interference with the spontaneous process of 
embryological development or interference 
with the biological conditions that sustain the 
108 
life of the embryo is the most certain kind of 
biogenetic control. A gametocide is a pirate 
on the high seas; an embryocide is a practical 
buccaneer who lays in wait atthenarrows. The 
embryo is surer prey than the gamete and the 
trade routes are best left undisturbed. 
Up to this point we have been concerned with 
gametogenesis, an evolutionary conservative 
process seemingly similar in mammal, bird, 
and fish and virtually identical within genera, 
families, and orders, There is little opportu- 
nity for selectivity in attacking a conserva- 
tive characteristic, But in embryogenesis and 
in the lifelines that nurture the embryo, there 
is great diversity even between closely related 
species, Selective control is made easier by 
distinguishing between a placenta and an egg- 
shell, between a zonary placenta and a discoidal 
placenta, between holoblastic and meroblastic 
yolk cleavage, between parental dependence 
and independence. Turn off the incubator to 
kill the sparrow egg; turn it ontokill the trout 
egg. 
In mammals, abortions or resorptions of 
embryos and fetuses occur spontaneously and 
can be induced in several ways. Medical and 
veterinary abortifacients are available on the 
shelf, Techniques for their administration to 
wildlife have been and are being worked out 
by Balser (1964) at Denver. 
Landy (1962) and Coppinger (1962) of the 
Massachusetts unit have reviewed the physio- 
logical inhibition of embryogenesis and embryo 
sustentation in birds. 
In chickens a great many drugs and feed 
contaminants have a statistically noticeable 
effect on hatchability of eggs. Gossypol in 
crude cottonseed oil or meal was found to re- 
duce hatchability by Bird (1956). The oil of 
the Java olive and cyclopropene fatty acids 
found in many malvaceous plants (Sterculi 
foetida) suppress hatchability (Schneider et al., 
1961). Any of the nutritional or pharmacological 
agents that affect quality of yolk, albumen, or 
shell tend to depress hatchability. We have 
already mentioned the fungicide Arasan caus- 
ing the production of soft-shelled eggs that 
will not sustain embryos. For one reason or 
another scores of these candidate compounds 
have been eliminated from our screening tests 
with the quail; they are not available commer- 
cially, or they are too expensive to produce, 
or they are apparently detoxified, or, as with 
