26 CLEAVAGE AND THE GERM LAYERS 



Cleavage in Amphibia.- These ova contain so much yolk that the 

 nucleus and most of the cytoplasm lie at the upper, or animal pole. The 

 first cleavage spindle appears eccentrically in this cytoplasm. The first two 

 cleavage planes are vertical and at right angles, and the four resulting cells 

 are nearly equal. The spindles for the third cleavage are located near the 

 animal pole, and the cleavage takes place in a horizontal plane. As a result, 

 the upper four cells are much smaller than the lower four (Fig. 15 A). 

 The large, yolk-laden cells divide more slowly than the upper, small cells 

 (B-D). At the blastula stage, the cavity is small, and the cells of the 

 vegetal pole are each many times larger than those at the animal pole (E,F). 

 The cleavage of the frog's ovum is thus complete but unequal. 



E F G 



FIG. 15. Cleavage and gastrulation in the frog. Xi2. A-D, cleavage stages ; E, blastula; 

 F, blastula in median section; G, early gastrula; H, median section of stage G. an., Animal 

 cells; arch., archenteron; b'c., blastocoele; b'p., blastopore; ect., ectoderm; ent., entoderm; 

 v 'S-> vegetal cells. 



Cleavage in Reptiles and Birds. The ova of these vertebrates contain 

 a large amount of yolk. There is very little pure cytoplasm except at the 

 animal pole, and here the nucleus is located (Fig. 3). When segmentation 

 begins, the first cleavage plane is vertical but the inert yolk does not cleave. 

 The segmentation is thus meroblastic, or incomplete. In the hen's ovum, 

 the cytoplasm is divided by successive vertical furrows into a mosaic of 

 cells, which, as it increases in size, forms a cap-like structure upon the 

 surface of the yolk (Fig. 16 A). These cells are separated from the yolk 

 beneath by horizontal cleavage furrows, and successive horizontal cleav- 

 ages give rise to several layers of cells (Fig. 16 B). The space between 

 cells and yolk mass may be compared to the blastula cavity of Amphioxus 

 and the frog (Fig. 18). The cellular cap is termed the germinal disc, or 

 blastoderm. The yolk mass, which forms the floor of the blastula cavity 



