572 MANUAL OF ZOOLOGY. 



as much as 103 to 104. The blood-corpuscles are oval and 

 nucleated. 



The respiratory organs are in the form of spongy cellular 

 lungs, which are not freely suspended in pleural sacs ; and the 

 bronchi open on their surface into a number of air-sacs, placed 

 in different parts of the body. 



All birds are oviparous, none bringing forth their young 

 alive, or being even ovo-viviparous. All birds are, lastly, pro- 

 vided with an epidermic covering, so modified as to constitute 

 what are known as feathers. 



Professor Huxley's account of the method in which feathers are produced 

 is so remarkably clear, that no apology is necessary for quoting it in its 

 entirety. Feathers "are evolved within sacs from the surface of conical 

 papillae of the dermis. The external surface of the dermal papilla, whence 

 a feather is to be developed, is provided upon its dorsal surface with a 

 median groove, which becomes shallower towards the apex of the papilla. 

 From this median groove lateral furrows proceed at an open angle, and 

 passing round upon the under surface of the papilla, become shallower, 

 until, in the middle line, opposite the dorsal median groove, they become 

 obsolete. Minor grooves run at right angles to the lateral furrows. Hence 

 the surface of the papilla has the character of a kind of mould, and if it 

 were repeatedly dipped in such a substance as a solution of gelatine, and 

 withdrawn to cool until its whole surface was covered with an even coat of 

 that substance, it is clear that the gelatinous coat would be thickest at the 

 basal or anterior end of the median groove, at the median ends of the late- 

 ral furrows, and at those ends of the minor grooves which open into them ; 

 whilst it would be very thin at the apices of the median and lateral grooves, 

 and between the ends of the minor grooves. If, therefore, the hollow cone 

 of gelatine, removed from its mould, were stretched from within, or if its 

 thinnest parts became weak by drying, it would tend to give way, along 

 the inferior median line, opposite the rod-like cast of the median groove, 

 and between the ends of the casts of the lateral furrows, as well as between 

 each of the minor grooves, and the hollow cone would expand into a flat 

 feather-like structure, with a median shaft, and a ' vane ' formed of ' barbs ' 

 and 'barbules.' In point of fact, in the development of a feather, such a 

 cast of the dermal papilla is formed, though not in gelatine, but in the horny 

 epidermic layer developed upon the mould, and as this is thrust outwards, 

 it opens out in the manner just described. After a certain period of growth 

 the papilla of the feather ceases to be grooved, and a continuous horny 

 cylinder is formed, which constitutes the ' quill.' " 



A typical feather (fig. 320) consists of the following parts : 

 i. The " quill " or " calamus " (a), which forms the basal por- 

 tion of the feather, by which it is inserted in the skin on its 

 own dermal papilla. It is the latest-formed portion of the fea- 

 ther, and consists of a hollow horny cylinder. 2. The " shaft " 

 or " rachis " (<), which is simply a continuation of the quill, 

 and which forms the central axis of the feather. The inferior 

 surface of the shaft always exhibits a strong longitudinal groove, 

 and it is composed of a horny external sheath, containing a 



