AVES BIRDS. 619 



lining membrane varies in texture in different parts : near the in- 

 fundibular orifice it is thin and smooth ; further down it becomes 

 thicker and corrugated ; and at last, near the termination of the 

 canal, where the egg is completed by the calcification of its out- 

 ward covering (g), it presents a villose texture. The oviduct 

 ultimately opens into the corresponding side of the urethro-sexual 

 compartment of the cloaca. 



(692.) We must, in the next place, proceed to describe, with as 

 much brevity as is consistent with the importance of the subject, 

 first, the nidus, or ovisac, in which the rudiment of the future being 

 is produced; secondly, the structure of the germ (ovulum) when 

 it escapes from the ovary ; thirdly, the additions made to the 

 ovulum as it passes through the oviduct ; and, lastly, the pheno- 

 mena that take place during the developement of the embryo by 

 incubation. 



(693.) If the ovarium of a bird be examined whilst in functional 

 activity, such of the pedunculated ovisacs (calyces, Jig. 283, f) as 

 have within them ovula ripe for exclusion, will be found to consist 

 of two membranes.* Of these, the exterior is very vascular, and 

 is surrounded with a pale zon& (stigma) , occupying the centre of the 

 calyx. The lining membrane of the ovisac, on the contrary, is thin 

 and pellucid, but studded with minute corpuscles, which are probably 

 glandular, or perhaps little plexuses of vessels. Within this 

 ovisac the basis of the future egg (ovulum) is formed. 



(694.) The ovulum produced in the ovisac, when mature, is 

 made up of the following parts. The bulk of it consists of an 

 orange-coloured oleaginous material, enclosed in a most delicate and 

 pellucid membrane (membrana vitelli ) ; this is the yolk of the 

 future egg. Upon the surface of the yolk there is visible a slightly 

 elevated opaque spot (cicatricula), wherein is lodged the repro- 

 ductive germ : this last, which is apparently the most important 

 part of the ovulum, is a minute pellucid globule ; and has been 

 named after its discoverer the " vesicle of Purkinje" or the 

 germinal vesicle. 



(695.) The phenomena attending conception are therefore sim- 

 ply these : The membranes of the ovisac are gradually thinned by 

 absorption ; and, being embraced and squeezed by the infundibular 

 commencement of the oviduct, the transparent zone or stigma gives 

 way, allowing the ovulum, covered only by its membrana vitelli, to 

 escape into the oviductus. The rent ovisac is soon removed by 

 * Vide Purkinje, Symbolae ad ovi Avium historiam ante incubationera. 4lo. Lipsise,1830. 



