REPRODUCTION AND DEVELOPMENT IN CHORDATES 



1st polar body 



Sperm nucleus 



Sperm path 



2nd polar body 

 Egg nucleus 



Germ 

 ring 



Gray 

 crescent 



'Epiboly 

 Involution 

 \ Dorsal lip 

 Blastopore 



Chordamesoderm 

 Gastrocoel ^ ^k'^!?^"''^':^^ Germ 

 Anterior .^,-i:>'-n—-^=i^-%J^nS ' 

 end 



N Ventral 

 surface 



Chordamesoderm 

 Ectoderm 



Mesoderm Neural plate 

 Endoderm 



Notochord Neural folds 

 Gastrocoel 



Endoderm 



Rg. 5.15. Early development in the frog. A, egg before activation, in section. B, one- 

 cell stage before amphimixis, in section. The first cleavage furrow would pass in the plane of 

 the paper, through the egg axis, and bisect the gray crescent. C, late blastula, in section. 

 D, early gastrula, in section. E, gastrula in section. F, late gastrula, in longitudinal 

 section, oriented to show dorsoventral and anteroposterior axes. G, H, and /, embryos during 

 the formation of the notochord, the dorsal mesoderm, and the neural plate, in cross section. 

 All figures diagrammatic; the arrow indicates the egg axis. {E and F, redrawn with modifica- 

 tions from R. S. McEwen, Vertebrate Embryology, copyright 1931 by Henry Holt and Co., printed 

 by permission. ) 



149 



