DEVELOPMENT OF THE CHICK. 



613 



and are caught by the dilated end of the oviduct. The 

 first part of the duct is narrow, and there the ova may be 

 fertilised ; the second part is wide and glandular, secreting 

 the white of egg ; in the third region, which is muscular and 

 glandular, the shell of the egg is made. 



How the shell is made we do not precisely know, 

 but it seems certain that it is not by the mechanical apposi- 

 tion of the secretions of the oviduct. A rudiment is present 

 from the first, and this rudiment uses materials provided by 

 the oviduct, and, although not cellular, grows by " intus- 

 susception." 



In sexual union the cloaca of the male is closely apposed 

 to that of the female ; only in a few cases (in ducks and 

 geese, and in the Ratitae), is there a copulatory organ. 



Development of the Chick. 



The ovarian ovum of the hen is a large spherical body, consisting 

 largely of yolk, but exhibiting at one region a disc of formative proto- 

 plasm with a large nucleus or ger- 

 minal vesicle. The ripening of the 

 egg is accompanied by the dis- 

 appearance of the nuclear mem- 

 brane, and also, judging from 

 analogy, by the formation of polar 

 bodies ; but the details of the 

 process are still obscure in the hen. 

 Either before it leaves the ovary, 

 or in the upper part of the oviduct, 

 the egg is fertilized by a spermato- 

 zoon. During the rest of its passage 

 down the female ducts, it undergoes 

 two sets of changes. On the one 

 hand it becomes surrounded by 

 various envelopes added to the deli- 

 cate vitelline membrane with which 

 it is already invested, on the other the 

 process of segmentation goes on 

 rapidly in the formative area. As a 

 result of these processes, we find that when laid the egg is surrounded 

 first by a firm porous shell of carbonate of lime, formed in the lowest 

 part of the oviduct ; beneath this there is a double shell membrane, the 

 two layers of which are separated at the broad end of the shell to form 

 an air chamber. This chamber grows larger as development proceeds, 

 and is of some importance, in connection with respiration, as an inter- 

 mediate region between the embryo and the external medium. Beneath 

 the shell membranes lies the albumen, or "white of egg," which is 

 secreted by the thin-walled region of the oviduct ; in it lie two spirally 

 twisted cords or chalazse, produced by the rotation of the egg in the 



FIG. 217. Diagrammatic sec- 

 tion of Egg. (After ALLEN 

 THOMSON.) 



g.v., Position of germinal vesicle ; 

 a.c., air chamber ; K., yolk (alternate 

 layers of "yellow" and "white"); 

 ch. chalaza. 



