152 



GENERAL PRINCIPLES OF ZOOLOGY. 



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 ordinary and more primitive mode, the total 

 cleavage; further, the eggs which show a partial cleavage are called 

 meroblastic, because only the segmented part of the egg is directly 

 employed in the formation of the embryo or bud (fiXacrTos), 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. The arrangement of the yolk is 

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

 maintains a central position and collects the yolk concentrically 

 around itself (centrolecithal eggs) (fig. 97), or it is pushed, together 



FIG. 97. 



FIG. 98. 

 71, nucleus; p, portion of the egg 



FIG. 97. Centrolecithal egg. (After O. Hertwig.) 



rich in protoplasm; d, portion rich in yolk. 

 FIG. 98. Telolecithal egg. (After O. Hertwig.) Letters as in fig. 97. 



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 nuclear pole, in the course of development, always 

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

 animal part rich in protoplasm and a vegetative part 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 portion. This condition 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 deutoplasm, upon which rests 

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



