THE CELL-DOCTRINE 



125 



sj, c V n 



- - a 3 ;.,l.L j J. H a&c/ 



P A 



A 



gastrula; H, early embryo; /, late embryo with dotted outline of an early 

 tadpole stage. The arrow passing through the vertical axis and polar 

 bodies of the egg in A and B is drawn in same position with reference to 

 parts of egg and embryo in G, H, and /. The poles of the unfertilized egg 

 can thus be traced to regions of the embryo and thence to the adult. 

 A and P, D and V, in A and / show the anterior-posterior and dorso- ventral 

 areas appearing in egg and tadpole respectively. Note rotation of egg 

 axis which occurs in stages represented by H, bringing dorsal surface to 

 its definitive position in /. Bp, blastopore (opening to primitive gut 

 cavity); c, notochord (primitive backbone); e, enteron (gut cavity); en, 

 endoderm (lining of gut cavity); m, mesoderm; mo, mouth; n, nervous 

 system; n cf and n 9, male and female pronuclei of oosperm; s, sper- 

 matozoon (nucleus entering egg, flagellum remaining outside); sc, seg- 

 mentation or blastula cavity. (Redrawn with modifications from Conklin, 

 "Heredity and Environment.") 



During the last fifty years, the cellular basis of develop- 

 ment and hence of heredity has been ascertained in mar- 

 velous detail. The adult features of animals have been 

 traced back to their origin in a few cells or even in single 

 cells. The egg-cell has been studied before and after fertili- 

 zation; and the cell-lineage of the adult parts has been made 

 known. In the frog, for example, the unfertilized egg ex- 



