SS THE PHYSIOLOGY OF TWINNING 



of chick twins are of very unequal size. The large 

 embryo is quite normal and has seventeen somites. The 

 small embryo, which is quite separate as yet from the 

 large one, is in about a four-somite stage and is therefore 

 some eighteen hours less advanced. The head end of the 

 small embryo nearly touches the body of the large one 

 and its axis is at right angles to the latter. The vitelline 

 circulation of the large embryo is seen to be invading the 

 vitelline area of the small one and it is highly probable 

 that the small embryo would later have become a mere 

 cyst on the side of the larger or might have been 

 totally absorbed. If one were to attempt to assign this 

 double embryo to one of the above-mentioned categories 

 he would be compelled to select number 2, for there is 

 no evidence that the two are derived from separate 

 blastoderms. These twins are, therefore, the product of 

 unequal double gastrulation. 



THE CAUSES OF TWINNING IN BIRDS 



Stockard points out that: 



The eggs of birds normally have a discontinuous mode of 

 development. Fertilization takes place in the upper part of the 

 oviduct 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 accessory coats. 

 Finally at the time of laying, the blastoderm has passed the 

 gastmla stage. The fall in temperature experienced on leaving 

 the body of the mother causes development to stop in this early 

 post-gastrula condition, and the egg remains quiescent until the 

 temperature is again raised to about that of the bird's body. 



It is believed that the reason why the sudden and 

 rather prolonged interruption of development so seldom 

 results in twinning is that the critical period for twin- 



