204 



VERTEBRATE ZOOLOGY 



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



tilization and ends with hatching. 



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



tion of the process of metamorphosis. 



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 maturation. 



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



eggs are laid in a string, 

 attached to one another 

 by 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 

 jelly, which is laid down 

 in two layers, has the 

 double function of con- 

 serving heat for incuba- 

 tion purposes and of pre- 



venting the 

 FIG. 118. Frog's egg before and after fertil- bacteria, 

 ization, showing symmetry relations. A, unfer- 

 tilized egg, from side; B, Unfertilized egg, from 

 vegetal pole; C, Fertilized egg just before cleav- 

 age 



attacks of 



The fertilized egg (Fig. 

 118) is rather highly 



from side; D, same from vegetal pole, organized before cleavage 

 u, gray crescent; p. pigmented animal pole; u . ,. ., 

 w, unpigmented vegetal pole. (From Kellicott.) be S ms > for the various axes 



of the future embryo 

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

 are already clearly defined. These relations can be made out readily 

 from the pigment pattern of the peripheral 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 other, separates the pigmented area from the pale yellow 

 area at the vegetal pole. Only one line can be drawn around the egg 

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

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



