DEVELOPMENT OF THE LEOPARD FROG 



405 



Fig. 213. — Development of the frog, showing orientation of the zygote in 

 relation to that of the tadpole as it appears in the median plane. 



The polar axis of the egg is shown by the arrow. It will be noted that this 

 shifts almost 180 degrees in the development from A to /. A and B, entrance 

 of spermatozoon and beginning of differentiation of cytoplasm into areas that 

 foreshadow certain adult structures. C, approach of egg and sperm nuclei 

 and further differentiation of the cytoplasm. D, two-cell stage. E, eight- 

 cell stage. F, early blastula. G, gastrula. H, transition from late neural- 

 fold stage to early embryo. 7, early embryo. 



A and P, D and F, anterior-posterior and dorso-ventral axes; bp, blastopore; c, noto- 

 chord; e, archenteron; ec, ectoderm; en, endoderm; to, mesoderm; mo, mouth; n, nerve 

 chord; 9 n, nucleus of ovum; cf". nucleus of spermatozoon; p.b., polar bodies; s, sper- 

 matozoon; sc, segmentation cavity or blastocoele; y.c, yolk-laden cells. (Modified from 

 Conklin, "Heredity and Environment," copyright, 1916, by Princeton University Press, 

 printed by permission.) 



