GENERA L EMBK YOLOGY. 



'S3 



only passively divided through the activity of the proto- 

 plasm in the cleavage cells. The more the mass of this 

 ballast increases in relation to the protoplasm, so much 

 the more slowly does the cleavage process proceed. 

 Finally there comes a point where the resistance of the 

 yolk becomes so great that the protoplasm is no longer 

 able to completely carry out the work ; then only the 

 protoplasmic part of the egg is divided, that which is rich 

 in yolk remaining an undivided mass. In this case, one 

 speaks of a partial cleavage in comparison with the ordi- 

 nary and more primitive mode, the total cleavage ; further, 

 the eggs which show a partial cleavage are called mcro- 

 blastic, because only the segmented part of the egg is 

 directly employed in the formation of the embryo or bud 

 (/?A<TTOS), while the undivided main mass serves merely as 

 food-material in the course of growth. Eggs with total 

 cleavage, on the contrary, are called holoblastic. 



Distribution of the Yolk. As to the second point, 

 the arrangement of the yolk : this is connected with the 

 position of the nucleus; either the egg-nucleus maintains 



P 

 d 



- n 



FIG. 94. FIG. 95. 



FIG. 94. Centrolecithal egg. (After O. Hertwig.) , nucleus ; /, portion of the egg rich in 



protoplasm ; d, portion rich in yolk. 

 FIG. 95. Telolecithal egg. (After O. Hertwig.) , nucleus; /, portion of the egg rich in 



protoplasm ; d, portion rich in yolk. 



a central position and collects the yolk around itself in 

 concentric arrangement (centrolecithal eggs] (Fig. 94), or it 

 is pushed, together with the greater part of the protoplasm, 

 to one pole of the egg, while at the other pole the yolk 

 predominates (telolecithal eggs). Since the nucleus-con- 



