REPRODUCTION. 



69 



described case, but the 

 behavior of the cells of 

 the blastosphere may be 

 hampered by a burden 

 of relatively foreign 

 matter, in the form of 

 food-yelk, in certain in- 

 stances ; so much so is 

 this the case that dis- 

 tinct modes of gastrula 

 formation may be rec- 

 ognized as dependent on 

 the quantity and ar- 

 rangement of food-yelk. 

 These we shall pass by 

 as being somewhat too 

 complicated for our pur- 

 pose, and we return to 

 the egg of the bird. 



The Hen's Egg.— By 

 far the larger part of 

 the hen's egg is made 

 up of yelk ; but just 

 beneath the vitelline 

 membrane a small, cir- 

 cular, whitish body, 

 about four millimetres 

 in diameter, which al- 

 ways floats uppermost 

 in every portion of the 

 egg, may be seen. This 

 disk (blastoderm, cica- 

 tricula) in the fertilized 

 egg presents an outer 

 white rim (area opaca), 

 within which is a trans- 

 parent zone (areapellu- 

 cida), and most centrally 

 a somewhat elongated 

 structure, which inarks 

 off the future being 

 itself (embryo). All 



Fig. 67.— Female generative orgasB of the fowl 

 (after Dalton). A, ovary; 3, Graafian follicle, 

 from which the egg has just heen discharged; 



C, yelk, entering upon extremity of oviduct; 



D, E, second portion of oviduct, in whicli the 

 chalazif erous membrane, chalazee. and albnmen 

 are formed ; ^, third portion,ln which the fibrous 

 shell membranes arc produced; G, fourth por- 

 tion laid open, showing the egg com^jletely 

 formed with its calcareous shell; H, canu 

 through which the egg la expelled. 



