i THE EGG 27 



rarely we find only a very small quantity equally distributed through- 

 out the protoplasm (holoblastie alecithal eggs, Fig. 23, C). Such 

 eggs are found among animals which can at a very early stage of 

 development find their own food, or else among those whose embryos 

 are developed within, and nourished by, the mother body. 



In most cases the egg contains a considerable quantity of nutritive 

 yolk. Two different types of eggs are distinguished, according to the 

 arrangement and position of this yolk. 



I. Types of Teloleeithal Eggs. 



A. In the simplest case the comparatively small quantity of nutri- 

 tive yolk is imbedded in the formative yolk, principally in one hemi- 



D 



'"">-_ 



"* x , 



FIG. 23. Structure of different eggs. A and B, Holoblastie telolecithal eggs. C, Holo- 

 blastie alecithal egg. D, Centrolecitlial egg (of a spider). E, Meroblastic telolecithal egg. 

 dp, Deutoplasm ; n, nucleus, or germinal vesicle. 



sphere. This hemisphere is called the vegetative ; the other, which 

 contains the germinal vesicle, the animal hemisphere. Corresponding 

 animal and vegetative poles are also distinguished. 



B. The quantity of the nutritive yolk increases so much that the 

 formative yolk is reduced to a smaller or larger segment at the animal 

 pole, in which lies the germinal vesicle. Besides this a thin layer of 

 protoplasm spreads all round the egg as a rind. In the remaining 

 portion of the egg the formative yolk is so much displaced by the 

 development of the nutritive yolk, that it remains merely as a cement- 

 ing substance in the interstices between the elements of the latter 

 (holoblastie teloleeithal eggs, Fig. 23, A). 



