6 



INTRODUCTORY 



supporting skeletal rods quickly lengthen, and the young 

 Pluteus assumes its ' easel ' shape (Fig. 2). Into the later deve- 

 lopment of additional arms, ciliated epaulettes and so on, and 

 into the formation of the body of the urchin on the left side 

 of the Pluteus, we need not stay now to inquire. 



As a second example we shall take another type whose egg 

 has been the subject of frequent and indeed classical experi- 

 ments, the Frog. 



The full-grown but unfertilized egg of the Frog is a spherical 



FIG. 2.- 



Pluteus of Echinus microtuberculatus from in front and from 

 the side. (After Boveri, 1896.) 



body, with a polarity which depends on the disposition of 

 yolk and cytoplasm, on the arrangement of the pigment, and 

 on the position of the nucleus. The egg is telolecithal ; that 

 is, the yolk is so disposed that the yolk-granules on one side 

 are larger and more numerous, on the other side smaller and 

 less numerous, and consequently the cytoplasm more abundant 

 on the latter, less so on the former side ; the distinction between 

 the two being, however, not sharply marked but gradual. There 

 is in fact a gradual increase of cytoplasm in passing from one 



