214 



PRINCIPLES OF ANIMAL BIOLOGY 



the oviparous and ovoviviparous vertebrates (reptiles, birds) , on the con- 

 trary, the eggs are very large. The yolk material is also very differently 

 distributed in the eggs of different species. With respect to the distri- 

 bution of yolk there are recognized three classes of eggs which are of 

 course connected by all gradations. They are as follows: 



1. Homolecithal eggs are those in which the yolk is nearly uniformly 

 scattered through the cytoplasm. Usually the amount of yolk is not 

 very great. The echinoderms (sea-urchins, starfishes), marine worms 

 and mammals produce eggs of this kind (Fig. 165). 



2. In telolecithal eggs the yolk is massed toward one side, the vegeta- 

 tive pole, leaving the bulk of protoplasm at the opposite side, the animal 





Fig. 165. — Homolecithal egg of the sandworm Nereis, c, cytoplasm; /, fat droplets; 

 m, egg membrane; n, nucleus; nl, nucleolus; y, yolk spheres. (After Wilson, courtesy of 

 Macmiilan Co.) 



pole. The yolk is abundant in such cases. The eggs of fishes, reptiles, 

 and birds are strongly telolecithal, while those of tTie frog are mildly so 

 (Fig. 166). 



3. Centrolecithal eggs have the yolk occupying the central portion, 

 while the greater part of the cytoplasm is in a layer at the peripiiery. 

 There is usually also an island of cytoplasm in the middle of the yolk 

 mass, in which the nucleus of the cell is located. Insect eggs are regularly 

 of this type (Fig. 167). 



Eggs are very often enclosed in a membrane or shell, particularly 

 among species that lay their eggs on land where evaporation must be 

 largely prevented. These envelopes may be of a chitinous nature, as 



