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 ovidueal 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 
