EMBRYOLOGY 



215 



among insects, or composed of keratin which resembles chitin, or they 

 may be impregnated with calcium salts. The shell of the egg of the 

 domestic fowl is composed of three layers. The inner layer is composed 

 of calcareous particles with conical faces pointing inward. These 

 particles do not fit closely, and air may pass between them. Outside 

 of this layer is a compact sheet of calcareous strands which also permits 

 the passage of gases. On the outer sur- 

 face of the shell is a third layer, the 

 cuticle, which appears to be structureless 

 except that it is penetrated by pores. 

 Within the shell is a membrane con- 

 sisting of two layers of fibers crossing 

 one another in various directions. The 

 envelope as a whole is calculated to 

 prevent excessive evaporation, a pro- 





Fig. 166. — Telolecithal egg in generalized 

 form, a, animal pole; c, cytoplasm; ?/i, second 

 maturation spindle; p, first polar bod> ; s, 

 spermatozoon; o, vegetative pole; y, yolk 

 crowded toward vegetative pole. (Modified 

 from original drawing by P. 0. Okkelberg.) 



Fig. 167. — Centrolecithal egg of the 

 fly Musca, in longitudinal section, cy, 

 cytoplasm; em, egg membrane; m, micro- 

 pyle; n, egg and sperm nuclei; pb, three 

 polar bodies; y, yolk. {From Korscheit 

 and Heiaer, after Henking and Blockmann, 

 courtesy of Macmillan Co.) 



vision necessary in eggs laid in air, and yet it permits the passage of gases 

 necessary for the respiration of the embryo. Indeed, air begins to 

 penetrate the shell soon after the egg is laid, and accumulates in a 

 space between the two layers of the membrane within the shell at the 

 large end of the egg. 



The Spermatozoa. — Spermatozoa exhibit much greater variety of 

 form than eggs. Typically they consist of an enlarged anterior head, 



