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. There are different cases: (1) those in 

 which the contents of the enclosed and the enclosing eggs are 

 substantially normal, though of course the enclosing shell is 

 abnormally large. (2) 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; double- 

 yolked eggs and eggs with abnormal yolks are due to abnormal 

 ovarian conditions; enclosed eggs to abnormal oviducal condi- 

 tions, 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 



