AVES. 



minute spherules of the same nature as the smallest white yolk 

 spherules. 



Impregnation takes place at the upper extremity of the 

 oviduct. 



In its passage outwards the ovum gradually receives its acces- 

 sory coverings in the form of albumen, shell-membrane, and shell 

 (fig. 87). 



c7,.7. 



<?. 



FIG. 87. DIAGRAMMATIC SECTION OF AN UNINCUBATED FOWL'S EGG. 



(Modified from Allen Thomson.) 



bl. blastoderm ; w.y. white yolk. This consists of a central flask-shaped mass and 

 a number of layers concentrically arranged around it. y.y. yellow yolk ; v.t. vitelline 

 membrane ; x. layer of more fluid albumen immediately surrounding the yolk ; 

 w. albumen consisting of alternate denser and more fluid layers ; ch.l. chalaza ; a.ch. 

 air-chamber at the broad end of the egg. This chamber is merely a space left between 

 the two layers of the shell-membrane, i.s.m. internal layer of shell-membrane; 

 s.m. external layer of shell-membrane ; s. shell. 



The segmentation commences in the lower part of the ovi- 

 duct, shortly before the shell has begun to be formed. It is 

 meroblastic, being confined to the germinal disc, through the 

 full depth of which however the earlier furrows do not extend. 

 It is mainly remarkable for being constantly somewhat unsym- 

 metrical (Kolliker) a feature which is not represented in fig. 88, 

 copied from Coste. Owing to the absence of symmetry the cells 

 at one side of the germinal disc are larger than those at the 

 other, but the relations between the disc and the axis of the 



10 2 



