26 THE DEVELOPMENT OF THE CHICK 



Abnormal eggs are of two main kinds: those with more than 



one yolk, and enclosed eggs (ovum in ovo). Double-yolked eggs 



are obviously due to the simultaneous, or almost simultaneous, 



liberation of two yolks, and their incorporation in a single set of 



egg-membranes. The two yolks are usually separate in such 



cases and are derived, presumably, from separate follicles. But 



two yolks within a single vitelline membrane have been observed; 



such are in all probability products of a single follicle. Cases of 



three yolks within a single shell are extremely rare. The class 



of enclosed eggs includes those in which there are two shells, 



one within the other. In some cases the contents of the enclosed 



and the enclosing eggs are substantially normal, though of course 



the enclosing shell is abnormally large; in others the enclosed 



egg may be abnormal as to size (small yolk), or contents (no 



yolk). In all cases described, the enclosing egg possesses a yolk 



(Parker). Abnormal eggs of these three classes are of either 



ovarian or oviducal origin; doubled-yolked eggs and eggs with 



abnormal yolks are due to abnormal ovarian conditions; enclosed 



eggs to abnormal oviducal conditions, or to both ovarian and 



oviducal abnormalities. Assuming the normal peristalsis of the 



oviduct to be reversed when a fully formed egg is present, the 



egg would be carried up the oviduct a greater or less distance and 



might there meet a second yolk. If the peristalsis became normal 



again, both would be carried to the uterus and enclosed in a 



common shell. (For a fuller discussion of double eggs see G. H. 



Parker.) 



Ovogenesis. The ovogenesis, or development of ova, may 



be divided into three very distinct stages. The first stage, or 

 period of multiplication, is embryonic and ends about the time 

 of hatching (in the chick) ; it is characterized by the small size of 

 the ova and their rapid multiplication by division. The multi- 

 plying primitive ova are known as ovogonia. At the end of this 

 period multiplication ceases and the period of growth begins. 

 The ova, known as ovocytes of the first order, become enclosed 

 in follicles; the size of the ovum constantly increases and the 

 yolk is formed. The third period, known as the period of matura- 

 tion, is characterized by two successive exceedingly unequal 

 divisions of the egg-cell, producing two minute cells, the polar 

 globules, that take no part in the formation of the embryo, but 

 die and degenerate. The process of maturation begins in the 



