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 com 
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.f 
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. 
$ Pearl and Curtis. Studies in the Physiology of Reproduction of 
the Domestic Fowl. V. Data Regarding the Physiology of the Oviduct. 
Jour. Exp. Zool., 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. 428- 
429, 1909. 
t Pearl and Curtis. Loc cit. 
