742 AVES. 



(rtiembrana vitelli} : this is the yelk of the future egg. Upon the sur- 

 face of the yelk there is visible a slightly-elevated opaque spot (clca- 

 tricula), wherein is lodged the reproductive 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 Pur- 

 Jcinje," or the germinal vesicle. 



(2104.) The phenomena attending conception are therefore simply 

 these : The membranes of the ovisac are gradually thinned by absorp- 

 tion ; and being embraced and squeezed by the infundibular commence- 

 ment of the oviduct, the transparent zone or stigma gives way, allowing 

 the ovulum, covered only by its membrana vitelli, to escape into the ovi- 

 ductus. The rent ovisac is soon removed by absorption ; and the ovulum, 

 with its cicatricula, is left to be clothed with other investments : but 

 the germinal vesicle is now no longer to be seen ; its delicate covering- 

 having been, as Purkinje supposes, ruptured by the violence to which it 

 has been subjected. 



(2105.) It is during the passage of the ovulum through the canal of 

 the oviduct that it becomes enclosed in the other parts entering into 

 the composition of the egg : these are, the albumen, the chalazce, the 

 membrana putaminis, and the calcareous shell. 



(2106.) The albumen, or glairy fluid forming the white of the egg, 

 is secreted by the mucous membrane that lines the commencement of 

 the oviduct ; and being laid on, layer upon layer, gradually coats the 

 membrana vitelli. Some of the albumen meanwhile becomes inspissated, 

 so as to form an almost invisible membrane, the chalazce, which, being- 

 twisted by the revolutions of the yelk, as it is pushed forward in the 

 oviduct, is gathered into two delicate and spiral cords (fig. 371, c c), 

 whereby the yelk is retained in situ after the egg is completed. 



(2107.) The ovulum, now covered with a thick coating of albumen, 

 and furnished with the chalazae,at length approaches the terminal extre- 

 mity of the oviduct, where a more tenacious material is poured out : it 

 is here that the whole becomes encased in a dense membrane resembling 

 very thin parchment, called " membrana putaminis ; " and ultimately, 

 on arriving in the last, dilated portion of the canal (fig. 369, g), the 

 lining membrane of which secretes cretaceous matter, the shell is formed 

 by the gradual accumulation of extremely minute, polygonal calcareous 

 particles, so disposed upon the surface of the egg that imperceptible in- 

 terstices are left between them for the purpose of transpiration. 



(2108.) Thus, as the oviduct is traced from its infundibular com- 

 mencement, the different portions of it are seen successively to discharge 

 the following functions: the orifice of the infundibulum receives the 

 ovulum from the ovisac ; the succeeding portion, extending nearly three- 

 fourths of its entire length, secretes the albumen and the chalaza3 ; in 

 the next tract it furnishes the membrana putaminis ; and in the last 

 place, the shell ; after which, the complete egg is expelled through the 

 cloaca. 



