BIRDS. 351 



formed of a thinner fluid ; the second or inner layer of albumen is 

 present in greater quantity, is thicker, more tenacious and very 

 clear in colour. Above and below the yolk is a tortuous filament 

 attached to the yolk-membrane by a broad base, whiter also and 

 firmer than the rest of the albumen (the two chalazce or grandmes) . 

 At the obtuse extremity of new-laid eggs there arises, in conse- 

 quence of the separation from each other of the two laminte of the 

 white tough membrane which invests the inner surface of the shell, 

 a small space. This space is caused by the evaporation of the 

 albumen whilst the atmospheric air passes inwards through the 

 shell; it gradually increases during the brooding \ 



The development of the young bird proceeds, as is well known, 

 by means of brooding after the tg^ has been laid. The birds form 

 the only class of vertebrate animals of which all the species are 

 oviparous. Pathological cases, however, have occurred in which 

 eggs that have been retained in the abdominal cavity or in the 

 oviduct have been hatched by the warmth of the living body 2. 

 The warmth requisite for the development of the chick in the o^g^ 

 is about 100" Fahrenh. (85 — 40° C.) ; in a somewhat lower tem- 

 perature the development still proceeds, though more slowly; a 

 temperature above 44" C. causes the death of the chick. Since the 

 eggs that lie directly under the breast of the hen are necessarily 

 exposed to a greater heat than those placed at the edge of the nest 

 the hen moves the eggs after a time and places those in the middle 

 at the edge, and conversely, so that tlie development of the whole 

 proceeds contemporaneously. This development continues in the 

 brooded egg of the hen for about twenty-one days. As it proceeds 



1 J. C. Hehl Diss, inmtg. de nafura et tisu. aeris ovis avium incliiso, Tubings, 1 796 

 4to, and in Eeil's Archiv, 11. s. 496 — 500. Hence arises a diminution of weight in 

 the eggs, even when not brooded ; but the loss of weight is greater in brooded ego-s. 



The air of this cavity does not differ materially in the proportion of its o-ases from 

 that of the atmosphere; at first, however, it appears to contain more carbonic acid gas. 

 In irrespirable gases, or when precluded from the access of atmospheric air, the eggs 

 cannot undergo development. Fresh eggs can endure great cold, before beino- frozen • 

 if they have been once frozen and again thawed, or if the capacity to germinate be 

 destroyed, as, for instance, by a powerful electric shock, then they freeze much more 

 readily; see experiments of Paget to this effect, Philos. Transactions, 1850, Part r. 

 pp. 221 — 226. 



2 "Etiam Gallina vivos foetus peperit." Hallee Elem. Physiol, viii. p. 46; com- 

 pare also the examples of this in Tiedemann Zoologic, in. s. 145 — 147, 



