152 



GENERAL rillNCIVLES OF ZOOLOGY. 



of the egg is divided, that which is rich in yolk remaining an 

 undivided mass. In tliis case one sj)eaks of a partial cleavage in 

 comparison with the ordinary and more primitive mode, the total 

 cleavage; further, the eggs which show a 2)artial cleavage are called 

 mcrobiastic, becanse only the segmented i)i\.vt of the egg is directly 

 employed in the formation of the emln'yo or bud {/jXaa-roi), while 

 the undivided main mass serves merely as food-material in the 

 course of growth. Eggs with total cleavage, on the contrary, are 

 called Itolobliistic 



Distribution of the Yolk. — The arrangement of the yolk is 

 connected with the position of the nucleus; either the egg-nucleus 

 maintains a central jiosition and collects the yolk concentrically 

 around itself (ceufrolecithal egg-i) (fig. 97), or it is pushed, together 



Fig. ST. Fig. iis. 



Fig. !iT. — C'entrolecithal escK. (After O. Ilcrtwi^.) n, nucleus; j.', portion of the egg 



rich in protoplasm; d, portion ricli in yolk. 

 Fig. !l8,~Tclolecitbal egg. ' (After O. Hertwig.) Letters .is in fig. 97. 



with the greater part of the protoplasin, to one pole of the egg, 

 while at the other pole the yolk predominates {tcloleciflial egg>')- 

 Since the nuclear pole, in the course of development, always 

 becomes the animal pole, there can be distinguished in the egg an 

 animal part rich in protojilasm and a vegetative i)art rich in yolk 

 (fig. 98). In many telolecithal eggs the two regions pass gradually 

 into one another, but in others a distinct boundary separates an 

 almost purely protoplasmic animal portion from a yolk-containing 

 vegetative jiortion. This conditioit is well shown in the bird's egg 

 (fig. 99). Here only the yolk is to be regarded as an egg in the 

 embryological sense, while the white, the egg-membrane, and the 

 calcareous shell are only later depositions upon the surface of the 

 egg. The chief mass of the yolk is dcuto])la-sm, upon which rests 

 a thin layer of protoplasm, the germinal disc, always itppermost 



