204 



VERTEBRATE ZOOLOGY 



2. The period of embryonic development, which begins with fer- 



tihzation and ends with hatching. 



3. The larval period, which extends from hatching to the comple- 



tion of the process of metamoiphosis. 



4. The period of adolescence, extending from the end of metamor- 



phosis to sexual maturity. 

 1. The period of germ cell formation involves both ovogenesis 

 and spermatogenesis, together with the processes of matiu'ation. 

 These stages are quite regular and require no special comment. The 



eggs are laid in a string, 

 P -~^^ ^1^ attached to one another 



1 >y means of a continuous 

 gelatinous envelope, which 

 is at first dense and vis- 

 cous, but soon absorbs 

 sufficient water to cause 

 it to swell to several times 

 its original thickness. This 

 icily, which is laid down 

 m two layers, has the 

 douljle function of con- 

 scr\-ing heat for incuba- 

 tion purposes and of pre- 

 \'enting the attacks of 

 ])actcria. 



The fertilized egg (Fig. 

 118) is rather highly 

 organized before cleavage 

 Ijegins, for the various axes 

 of the future embryo 

 (antero-posterior axis, dorso-veutral axis and the axis of bilaterality) 

 are already cl(>arly defined. These relations can be made out readily 

 from the pigment pattern of the peri]iheral parts of the egg. The upper 

 hemisphere of the egg is covered with black pigment, which is like an 

 obliquely placed cap. A gray crescent, thick at one side and fading 

 out on the othci-, separates the pigmented ai-ea from the pale yellow 

 area at the vegetal pole. Only one line can be di-awn around the egg 

 so as to divide it into bilaterally eciual halves; this represents the 

 primary axis of the embryo. The yolk is abundant, and only a small 



B 



Fig. lis. — Frog's egp; before and after fertil- 

 ization, showing symmetry relations. A, unfer- 

 tilized egg, from side; Ji, Unfertilized egg, from 

 vegetal pole; C, Fertilized egg just before cleav- 

 age, from side; D, same from vegetal pole. 

 C, gray crescent; p, pigmented animal pole; 

 w, unpigmented vegetal pole. (From KcUicott.) 



