210 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



known among the vertebrates. The simplest conditions pre- 

 sented by these forms may be illustrated by the amphibia, 



the outlines which 

 follow being to a 

 measure true of Petro- 

 myzon and some gan- 

 oids. 



In the amphibia 

 the first two planes 

 of division may be 

 compared to two me- 



FIG. 214 Two stages in the segmentation of the 

 egg of Amblystoma; i to 5 the successive planes of 



segmentation. 



Q 



right angles to each 

 other. These begin 



to cut through at the protoplasmic (darker) pole of the egg, 

 and gradually extend to the other. The third plane is at right 

 angles to these, but nearer the protoplasmic than to the deu- 

 toplasmic pole. The result is that the eight resulting cells 

 are unequal in size, four being small and four much larger, 

 This disparity in size is continued in the following divisions, and 

 it also affects the position of the internal segmentation cavity 

 (p. 5), which, instead of being central, 

 is pushed toward the protoplasmic 

 pole (Fig. 215). In the amphibia, 

 then, the whole egg divides into cells. 

 Such eggs are called holoblastic. 



In elasmobranchs, reptiles, and 

 birds, where the deutoplasm is much 

 more abundant, and the polar differ- 

 entiation of the egg is more marked, 

 the planes of segmentation do not FIG. 215. Early stage of the 



CUt through the entire egg, but are segmentation of the egg of Am- 



blystoma, in section, showing the 

 excentric position of the segmen- 

 tation cavity, s. 



confined to what is called the ger- 

 minal area at the protoplasmic pole. 

 Here occur meridional and circular 

 planes of division, so that the germinal area is converted into 

 cells, while the bulk of the egg remains unsegmented. In 

 these meroblastic eggs the segmentation cavity is still farther 



