504 MANUAL OF ELEMENTARY ZOOLOGY 



The difference in the segmentation of the ova of the 



lancelet and the frog is due to the presence in 

 segmentation. t ^ ae latter of a considerable quantity of yolk 



or food material stored to provide for the 

 nourishment of the embryo during the early stages of 

 development. This yolk, lying on one side of the egg, 

 hampers the relatively scanty protoplasm there, so that it 

 divides more slowly. In the dogfish and in birds the yolk 

 is still more plentiful, with the result that the portion of 

 the egg in which it is stored never divides at all, but remains 

 as an inert mass until it is surrounded by the growth of 

 the small protoplasmic region or germinal disc which lies 

 originally at one pole containing the nucleus, and segments 

 to form the cells of the embryo. The segmentation of the 

 ovum of the lancelet is complete or holoblastic and almost 

 equal ; that of the ovum of the frog is holoblastic and unequal; 

 in the dogfish and in birds it is incomplete or meroblastic. 

 Segmentation of the egg of a bird, such as the common 



fowl, begins with the formation across the 

 of V t e he P Ch?ck. germinal disc of a furrow which does not quite 



reach its edge. This is soon crossed by another 

 furrow, and then more appear till the disc is divided 

 into a mosaic of small irregular segments (Fig. 384, 1). 

 Sections of the disc show that at the same time horizontal 

 clefts are forming by which the segments become separated 

 from the underlying yolk. By a further series of horizontal 

 clefts the disc then becomes two or three cells deep. In 

 this way, shortly before the laying of the egg, a cap of cells 

 known as the blastoderm is formed. In this (Fig. 384, 2) 

 the upper layer, the epiblast, is separated by a chink, the 

 blastocoele, from a deeper mass of lower layer cells or 

 primitive hypoblast, which will give rise to the hypoblast 

 and mesoblast. The first rudiment of the enteron 

 appears as a space formed by the separation of the 

 lower layer from the underlying yolk, known as the 

 sub-germinal cavity. In a surface view of the blastoderm 

 (Fig. 384, 3), this gives rise to a central translucent area 

 pellucida, round which is the area opaca, where the edge of 

 the blastoderm rests on the yolk. The blastoderm grows 

 over the yolk, the epiblast extending by the division of its 

 cells, the lower layer partly in this way, partly by the 



