Heredity and Environment 



V 



FIGS. 10, ii. DIAGRAMS OF FROG'S EGGS SHOWING THE RELATIONS OF THE 

 AXES AND SUBSTANCES OF THE EGG TO THE AXES AND PRINCIPAL ORGANS OP 

 THE EMBRYO. All eggs viewed from right side, polar bodies above ; A, 

 anterior; P, posterior; D, dorsal; V, ventral; s, spermatozoon; $N, sperm 

 nucleus, $ N, egg nucleus ; m, mesodermal crescent where mesoderm will 

 form; c and n, gray crescent, where chorda (c) and nervous system (n) 

 will form; en, area of endoderm; area around polar bodies will form ecto- 

 derm of skin; sc, segmentation cavity; bp, blastopore; E, enteron; M, 

 region of mouth. 



