STRUCTURE AND DEVELOPMENTAL RATE 187 



It is a well-known fact, as mentioned in the early pages of this 

 discussion, that the eggs of birds normally have a discontinuous 

 mode of development. Fertilization takes place in the upper 

 part of the ovdduct and the egg begins its development in the 

 high temperature of the maternal body and continues to develop 

 as it travels down the uterine tube and becomes surrounded by 

 its several accessory coats. Finally, at the time of laying, the 

 blastoderm has passed the gastrula stage. The fall in tempera- 

 ture experienced on leaving the body of the mother causes devel- 

 opment to stop in this early postgastrula condition, and the egg 

 remains quiescent until the temperature is again raised to about 

 that of the bird's body. 



From the evidence given in preceding sections regarding the 

 developmental time of inducing double embryo formations, it is 

 seen that the bird's egg at laying has just passed the critical 

 moment for causing double invagination or double blastopore 

 formation. Since these double invaginations may be brought 

 about by either interrupting development or slowing its rate 

 before gastrulation, it would seem that the bird's egg had been 

 piloted beyond this danger period within the body of the mother. 

 How, then, is the frequency of double and twin chicks in the bird's 

 egg to be accounted for? 



In studies on the early stages of development in the bird's egg, 

 it has been found by Patterson ('09) and others that the process of 

 gastrulation takes place very close to the actual moments of lay- 

 ing. The time relationships between the moments of laying and 

 finished gastrulation are, however, in general slightly variable, 

 and the eggs of certain females, as I learn in conversation with 

 Professor Patterson, differ decidedly from others in their tendency 

 to be deposited at an unusually early stage. There would thus 

 seem to be a strong probability that all eggs of the bird have not 

 reached or passed the gastrulation process before the time of 

 laying. This is a most important probability, and is believed to 

 be true by some of those who have studied these early stages very 

 extensively. 



On the basis of my own experimental results, this probable 

 variation in the moment of laying is entirely sufficient to account 



