438 



EMBRYOLOGY OF THE CHICK 



egg remains extremely small because the nourishment of the embryo is 

 derived directly from the mother. 



During the very first day of incubation the outlines of the embryo 

 are defined. During the second day a rather complicated series of folds 

 appear, separating the embryo from the yolk. The embryo, however, 

 remains in contact with the yolk-mass by a narrow stalk. The circula- 

 tory system now develops, through which nourishment is carried from 

 the yolk-mass to the embryo. Embryonic membranes and appendages 

 appear during the second and third days of incubation. These assist in 

 respiration and also in forming a larger area from which the food supply 

 may be brought from the yolk to the embryo. 



Development usually begins at the head end and extends tailward, 

 so that the brain and other head structures are often quite well devel- 

 oped when there is little semblance of any other well-defined structure 

 toward the tail end. The chick usually makes a small opening in the 

 egg shell on about the twentieth day of incubation, and from then on 

 the lungs actually take in air and begin their regular external work, while 

 on the twenty-first day the chick breaks through the shell entirely. 



With this introductory general outline, we shall take up the study 

 of the egg itself, working backward to its very simplest cell origin in 

 the mother's ovary. 



THE EGG 



The true ovum (Fig. 251, v.), or egg-cell proper, is the large yolk 

 or vitellum. This is surrounded by a tough vitelline membrane. The 



ad 



*/ 



Blastopore 

 (crescentic 

 groove) 



yy 



Fig. 251. 



Semidiagrammatic illustration of the hen's egg at the time of laying. A. 

 Entire "egg." B. Diagram of a vertical section through the vitellus or ovum 

 proper, showing the 'concentric layers of white and yellow yolk, a, Air chamber ; 

 ac, chalaziferous layer of albumen ; ad, dense layer of albumen ; a}, fluid layer of 

 albumen ; 6, blastoderm ; c, chalaza ; I, latebra ; nl, neck of latebra ; P, nucleus of 

 Pander ; pv, perivitelline space ; smi, inner layer of shell membrane ; smo, outer 

 layer of shell membrane ; v, vitellus or "yolk" ; vm, vitelline membrane ; wy, layers 

 of white yolk ; yy, layers of yellow yolk. C. Surface view of Blastoderm of un- 

 incubated hen's egg. (A and B. after Marshall ; C, after Hertwig.) 



