64 OUTLINES OF CHORDATE DEVELOPMENT 



III. THE LARVAL PERIOD. From hatching to metamorphosis. The 

 actual developmental processes of this and the latter part of the embry- 

 onic period (II, B.) are most conveniently described together. 



IV. THE ADOLESCENT PERIOD. From the beginning of metamorphosis 

 to sexual maturity. 



The durations of these periods can be given only in the 

 roughest way since they vary with the species and particularly 

 with the temperature and food supply; to a lesser extent the 

 same is true regarding size. The ages and lengths given 

 below are to be regarded then as _only approximations under 

 favorable conditions of development. 



The period of formation of the germ cells occupies the long 

 interval between the annual spawning seasons. For the most 

 part the germ cells are formed during the summer so that in the 

 following spring only the final steps remain to be accomplished. 

 Fertilization is external and maturation of the egg is not com- 

 pleted until the entrance of the sperm cell. Spawning occurs 

 during the first warm days of spring in most species, although 

 some, like the _bull-frog (R. catesbiand) may not spawn until 

 summer. The cleavage of the egg terminates with the forma- 

 tion of a fairly typical blastula, followed by gastrulation, which 

 is complicated during its later phases by the precocious proc- 

 esses of notogenesis and formation of the middle germ layer. 

 During these early phases of development, which usually 

 occupy about thirty-six hours, the spherical form of the egg is 

 retained (Fig. 22, B}, though a slight enlargement may result 

 from the formation of internal cavities and the absorption of 

 water. As notogenesis is completed the embryo begins to 

 form, bent around the curved surface of the gastrula (Fig. 22, 

 C, D}. Soon, however, the embryo becomes slightly elongated, 

 and shortly this elongation becomes quite marked chiefly as 

 the result of the enlargement of the head region and the growth 

 of the posterior part of the body (Fig. 22, E). Fig. 22, F, 

 shows an embryo of about two days (2.5 mm.), at the stage 

 which we may arbitrarily assume to represent the end of the 

 first division of the embryonic period. 



The neural tube is entirely closed, the blastopore roofed over, 



