STUDIES IN EMBRYONIC MORTALITY IN THE FOWL IV: COMPARATIVE MORTALITY RATES IN EGGS LAID A T DIFFERENT PERIODS OF THE DAY AND THEIR BEARING O N THEORIES OF THE ORIGIN OF MONSTERS

Various studies upon the development of eggs of fowls and of pigeons have shown that the processes of formation of the egg and of development of the embryo during the passage of the egg down the oviduct are continuous throughout the twenty-four hours of the day. They are involuntary reactions. On the other hand the actual laying of the egg is a reflex for which the necessary sensory stimulus usually does not occur while the hen is on the roost at night. This means that if an egg be fully formed and ready to lay at 2 A. M., it is retained in the uterus till the following morn-ing. The embryo of such an egg (when fertile) would pre-sumably have undergone some six hours further development than if the egg had been fully formed at 2 p. M. and had been laid at 2:30 p. M. Such a view is consistent with the observa-tions of Patterson1 who found considerable variation in the degrees to which embryos had developed by the time of laying. The problem arises of whether or not such variation in the development of the embryo at the time of egg laying, (when the egg is chilled and growth is arrested) affects its chances of surviving after it is put in the incubator. The work of Stockard2 oh eggs of Fundulus indicates that in eggs chilled at an early stage of gastrulation, or prior to initiation of that process, abnormalities in development are more likely to oc-cur, while eggs chilled after gastrulation is completed are bet-ter able to withstand the resultant arrest of growth and to con-tinue development normally when the necessary temperature is restored. Since Patterson3 and others have shown that in eggs of the fowl and of the pigeon gastrulation takes place very close to the time of laying, it is to be expected that in eggs laid a