SEC. IV ANATOMY OF BIRDS 331 



maturity, and the date of the initial impulse cannot well be deter- 

 mined. As there is probably but one ^^J.^i, at a time in the oviduct, 

 the whole process of finishing off the yelk-ball with its chalaziform, 

 soft albuminous, putaminous, and calcareous envelopes may go on 

 in twenty-four hours, most of which time is consumed in the shell- 

 formation. The number of eggs matured by the human female is 

 or should be thirteen annually ; this is no large number for many 

 of the gallinaceous and anatine birds to deposit in about as many 

 days. But a probable average number is five or six. Defeat of 

 the procreative instinct from any accident is commonly a stimula- 

 tion to renewed endeavours to reproduce ; and very many birds 

 rear two or three broods annually, though one clutch of eggs is the 

 rule. Many, such as auks, petrels, and penguins, lay a single egg. 

 Two eggs is the rule in humming-birds and pigeons. Three is 

 normal to gulls and terns, though these often have but two. Four 

 is the rule among the small waders of the limicoline groups. Some 

 of the small Oscines lay over the average, having eight or ten ; among 

 these, the European sparrow, Passer domesticus, is probably the most 

 prolific. The parasitic cuckoos are said to lay the relatively smallest 

 eggs ; that of the Apferijx is said to be the largest, weighing one- 

 fourth as much as the bird. The usual shape of an egg has given us 

 the common names oval, ovate, and ovoidal, for the well-known figure. 

 Some, as those of owls, woodpeckers, kingfishers, and others, more 

 or less nearly approach a spherical shape. Eggs of grebes, herons, 

 totipalmate birds and various others are rather elliptical, or equal- 

 ended, and narrow in proportion to their length. Eggs of the 

 limicoline group are generally pyriform, — very broad at one end 

 and narrow at the other. But the eggs of all birds vary more 

 in size and shape than some of the devotees of theoretical oology 

 admit in their practice. The variation so well known in any breed 

 of domestic foAvl is scarcely above a normal rate. The short 

 diameter, corresponding to the calibre of the oviduct, is less variable 

 than the long axis ; for when the quantity of food-yelk and white, 

 upon which the difference in bulk depends, varies with the vigour 

 of the individual, the scantiness or redundancy is expressed by the 

 shortening or lengthening of the whole mass. The e^g traverses 

 the passage small end foremost, like a round wedge, with obvious 

 reference to ease of parturition by more gradual dilatation of the outlet. 

 Germination. — Leaving now alL the accessory parts of an egg, 

 let us confine attention to the germ-yelk, or " tread," which is alone 

 concerned in the germinative process. Recurring to the female 

 Dynamanuela, consisting of granular protoplasm (vitellus) included 

 in its cell-wall (vitelline membrane) and including its nucleus and 

 nucleolus (germinal vesicle and germinal spot), we will trace it up 

 to the time it begins to take shajse as an embryo chick. At first, 



