DEVELOPMENT PRIOR TO LAYING 39 
other important phenomena are going on during the cleavage 
period. 
The type of cleavage exhibited by the bird’s egg is known 
as meroblastic, for the reason that only a part of the ovum is 
concerned, viz., the germinal disc. This is obviously due to the 
great amount of yolk (see Introduction, pp. 11 and 12). 
To understand the form and significance of the cleavage of 
the bird’s egg, it is necessary first of all to gain a clear idea of the 
structure of the germinal dise and its relations to the yolk. At 
the time of the first cleavage the germinal disc is round in surface 
view and about 3 mm. in diameter; the center is white and is 
surrounded by a darker margin about 0.5 mm. wide. These 
two zones have been compared to the pellucid and opaque areas 
of later stages, but it is certain that the correspondence is not 
exact. We shall call the outer zone the periblastic zone, or simply 
periblast. In section, the germinal disc is biconvex, but the 
outer surface which conforms to the contour of the entire egg 
is much less arched than the inner surface. The dise is every- 
where separated from the yellow yolk by a layer of white yolk 
(Fig. 2); on the other hand, there is no sharp separation between 
the dise and the white yolk. The granules of the latter are largest 
in the deeper layers and there is a gradual transition from them 
to the smaller yolk-granules with which the disc is thickly charged 
(Fig. 19). It is practically impossible in a section to say where 
the protoplasm of the disc ceases; it is indeed probable that it 
extends some distance into the white yolk both beneath and 
around the margins of the disc. Thus in Figure 21 a cone, ap- 
parently of protoplasm, extends into the neck of the latebra a 
considerable distance. In other cases it does not extend so far. 
The Hen’s Egg. The form of cleavage of the hen’s egg is 
illustrated in Fig. 16, A-E. The first cleavage appears in surface 
view as a narrow furrow extending part way across the germinal 
dise (Fig. 16 A). According to Coste the furrow is central in po- 
sition, but Kolliker describes it as excentric. Probably both con- 
ditions may be found in different eggs. While the ends of the 
first cleavage furrow are still extending towards the periblast, the 
second division begins. It is a vertical division in each cell like 
the first and the two furrows meet the first cleavage furrow at 
right angles. They may meet the first furrow at approximately 
the same point, in which case they form an approximately straight 
