EMBRYONIC DEVELOPMENT 



199 



The Eggs. — The eggs of animals are typically spherical or nearly so. 

 Often, however, one diameter is much greater than the others, or the 

 egg may be elongated and curved, as in many insects. Internally the 

 substance of the egg is in some way differentiated so that opposite 

 sides or poles do different things. One 

 side is known as the animal pole, the 

 opposite side as the vegetative pole. 



The food, or yolk, stored in an egg may 

 be very meager and is in such instances 

 rather uniformly distributed through the ^ 

 protoplasm. Sea urchins, marine worms 

 (Fig. 169), and mammals have such eggs. 

 In fishes, reptiles, and birds, and less 



Fig. 170. Fia. 171. 



Fig. 170. — Generalized egg of telolecithal type, a, animal pole; c, cytosome; m, 

 second spindle in oogenesis; p, first polar body; s, spermatozoon; v, vegetative pole; y, yolk 

 crowded toward vegetative pole. 



Fig. 171. — Centrolecithal egg of the fly Musca, in longitudinal section, cy, cytosome; 

 em, egg membrane; m, micropyle; 7i, egg and spermatozoan nuclei; pb, three polar bodies; y, 

 yolk. (From Korschelt and Heider, after Henking and Blockmann. Courtesy of The Mac- 

 ■millnn Company.) 



strikingly so in frogs, the yolk is crowded toward the vegetative 

 pole, so that most o*f the protoplasm is at the animal pole (Fig. 170). 

 In insects the yolk is in the central part, with a principal layer of 

 protoplasm at the surface (Fig. 171). Eggs with little yolk are said to be 

 alecithal or, from the uniform distribution of the yolk, homolecithal. Eggs 

 with much yolk aggregated toward the vegetative pole are telolecithal; 

 those with the yolk in central position, the protoplasm in a surface 

 layer, are centrolecithal. 



