SIZE, SHAPE AND PHYSICAL CONSTITUTION OF THE EGG. 1 33 



brane.t The membrane becomes thicker during the passage 

 through the isthmus * probably by the addition of successive 

 layers of secretion. 



When the egg enters the uterus it is enclosed in a firm tough 

 membrane and is often if not always already shaped. It con- 

 tains the yolk with chalazae and 60 to 70 percent of its albu- 

 men. § 



Within the uterus the egg receives the rest of its albumen 

 (by osmosis) and its shell. The nature of the stimulus which 

 sets up the shell secreting activity was investigated by Pearl and 

 Surface.* The conclusions from their preliminary experi- 

 ments were that the nature of the immediate stimulus which 

 sets the shell secreting activity going in an oviduct in active 

 functional condition is mechanical, and that shell formation is 

 a local reflex not immediately dependent upon a specific activ- 

 ity of other portions of the reproductive apparatus. 



That the weight of the shell is significantly correlated with 

 the weight of each of the other parts indicates that the amount 

 of secretion is influenced by the degree of stimulation. The 

 larger the egg the greater the mechanical stimulation and hence 

 the heavier the shell. The higher correlation between albumen 

 and shell than between yolk and shell is probably due almost 

 entirely to the fact that the albumen is heavier than the yolk. 

 It may be partly due to the fact that 30 to 40 percent of the 

 albumen is secreted in the uterus and that the periods of the 

 albumen secretion and shell secretion overlap although the 

 former is evidently complete before the latter has advanced 

 very far.t 



Abnormally thin shelled eggs are as likely to be large as 

 small and no doubt represent either a premature expulsion of 

 the egg or an early arrest of the shell secreting activity. 



i Pearl and Curtis. Studies in the Physiology of Reproduction of 

 the Domestic Fowl. V. Data Regarding the Physiology of the Oviduct. 

 Jour. Exp. Z06I., Vol. 12, 1912, pp. 99-124. 



'Membranes on eggs just completely past the cranial end of the 

 isthmus weigh .24 to .28 grams. Those at the caudal end .53 to .58 

 grams. 



§ Pearl and Curtis. Loc. cit. 



* Pearl, R. and Surface, F. M. Science, N. S.. Vol. XXIX, pp. 42S- 

 429, 1909. 



t Pearl and Curtis. Loc cit. 



