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 disc 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. 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 disc 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 disc 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 

 disc (Fig. 16 A). According to Patterson it occurs just as the egg 

 is entering the isthmus about three hours after the estimated time 

 of fertilization. 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 fur- 

 rows 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 hne (Fig. 16 B), or they 



