DEPARTMENT OF GENETICS. 121 



The results of these studies support and extend the conception that both 

 abnormahties and twins owe their origin to alterations of the developmental 

 rate, or perhaps the metabolic rate, at various and unequally susceptible 

 developmental stages. But whether a stage is "critical" or "indifferent" in 

 Stockard's sense depends in part, at least, upon its modifiability by a retarding 

 or an accelerating agent, and the stage of development at which twins or 

 multiple embryos actually arise extends all the way from the unsegmented 

 egg to gastrulation. 



Relation of Blood Sugar to Ovulation. 



Dr. Oscar Riddle, with the cooperation of Dr. H. E. Honeywell, has 

 shown that at each ovulation period there occurs a distinct suprarenal hyper- 

 trophy and concomitantly a rise in the concentration of the sugar of the 

 blood. It has seemed probable that measurements of the blood sugar might 

 throw light on the causes of diminished or suppressed ovulation. Studies 

 on this subject were undertaken both because of the practical and the the- 

 oretical importance of a knowledge of the factors which regulate egg produc- 

 tion. The practical importance is obvious, inasmuch as fecundity enters in 

 one way or another into most genetic studies. The theoretical importance 

 to our sex studies we have long recognized, since different sex-ratios are 

 obtained from pigeon clutches according as they are separated from pre- 

 ceding clutches by long or short intervals. 



A year ago Drs. Riddle and Honeywell reported (Year Book, 1922, p. 121) 

 that one condition that inhibits ovulation, namely, inactivity or confinement 

 in small space, reduces within 3 weeks the blood sugar of common pigeons 

 from 0.180 to 0.125 per cent. Such pigeons produce few or no eggs. During 

 the present year the same investigators have shown that the onset of cold 

 weather also causes a distinct fall in the blood-sugar values of several kinds of 

 pigeons. The duration of the initial decrease is, however, neither long nor 

 constant. This period of decreased sugar values is one of decreased ovulation 

 rate in our entire collection of birds. 



These points established, Dr. Riddle injected insulin into the pigeons; 

 this lowered the blood sugar and nearly all ovulations were inhibited. Pre- 

 cisely, the dose of insulin used lowered the blood sugar of ring-doves from 

 0.149 per cent to about 0.080 per cent during 1 to 4 hours twice daily. Under 

 this treatment only about one-tenth of the normal or expected number of 

 ovulations was realized. In the course of this study it was learned that ova 

 which have approached to within 48 to 72 hours of ovulation are not sup- 

 pressed by this dosage of insulin, and this led to a study of the effects of 

 insulin on the size of ova. These studies seem to establish the principle 

 that conditions which lower the concentration of sugar in the blood are 

 incompatible with ovulation. 



Death of Embryos in Eggs Abnormally Retained in the Oviduct. 



Various students of poultry have observed that an occasional fresh-laid 

 egg of the common fowl already contains a live or dead embryo of one, two, 

 or three days of growth. In these cases it is obvious that such eggs were 

 retained or delayed within the hen's oviduct during at least the period rep- 

 resented by the age of the embryo. Why some of these embryos are dead 

 and others alive is a question which seems never to have been investigated. 



In our own studies, and in all genetic work with birds, it may be of im- 

 portance to know the cause of failure or death in all eggs or embryos which 



