2IO MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



Fig. 214. Two stages in the segmentation of the 

 egg of Amblystovia; i to 5 the successive planes of 

 segmentation. 



known among the vertebrates. The simplest conditions pre- 

 sented by these forms may be illustrated by the amphibia, 



the outlines which 

 -, 1 follow being to a 



measure true oiPetro- 

 viyzon and some gan- 

 oids. 



In the amphibia 

 the first two planes 

 of division may be 

 compared to two me- 

 ridians of a globe at 

 right angles to each 

 other. These begin 

 to cut through at the protoplasmic (darker) pole of the eggj 

 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 di\isions, 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 ela^mobranchs, reptiles, and 

 birds, where the deutoplasm is much 

 more abundant, and the polar differ- 

 entiation of the z^^ is more marked, 

 the planes of segmentation do not 



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



confined to what is called the ger- ''"!•■''<"'"'• '" section, showing the 



1 ^ , i 1 • 1 excentiic position of the sesmen- 



mmal area at the protoplasmic pole, j^j;^^ cavity, s. 



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 



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Fig. 215. Early stage of the 



