THE CELL-DOCTRINE 



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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 7 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 7. 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 d" 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- 



