CHAPTER XII. 



GENERAL DISCUSSION. 

 A. RELATION OF HEREDITY AND ONTOGENY. 



In studying heredity our attention must often be focused on the onto- 

 genesis of the different characters, and we are sometimes incUned to regard 

 the adult character as the product of the course of ontogenesis. But this 

 is a superficial way of looking at things; the determiners of all characters 

 are in the germ-plasm and together they direct the development of one 

 part after another in orderly succession; a modernized form of the pre- 

 formation doctrine seems logically necessary. 



What do we know of the processes that take place in bringing the 

 fertilized egg, freighted with its specific heredity, to its destination — the 

 adult form? Modern embryological and cytological studies give us an 

 insight into many of them. First of all, the egg has a certain organization 

 that foreshadows something of its fate. Then cell-divisions begin, at first 

 synchronous, but later becoming accelerated here and retarded there. 

 Eventually (especially among animals) these cells become arranged into a 

 membrane whose unequal growth in limited areas produces foldings. The 

 folding of membranes, their stretching, local thickenings, or thinnings are 

 largely the result of local inhibitions of water. Sometimes movements of 

 individual cells occur out of the membranes into and through cavities or 

 solid yolk-masses, and by the aggregation of such cells massive organs are 

 sometimes formed. Local absorption of tissues already established may be 

 effected in later Ufe by such migratory cells. Membranes once established 

 may form pockets or linear folds, as in gastrulation and gland formation; 

 they may become perforated; two membranes may fuse along areas or 

 lines and a perforation may even occur at the region of fusion. Linear 

 strands or tubules may grow out, making connections, as nerves do, with 

 distant organs; tubes may unite to form a network, or split lengthwise. 

 Finally, membranes and masses undergo vacuoUzation, or masses may spUt 

 apart or fuse together. Thus in the ontogeny that is proceeding under the 

 control of heredity all is motion and change. 



What are the factors that control all these movements — ^for these are 

 the true factors of heredity? We do not know much about them, but we 

 know some things. We know that cell-divisions occur at particular times 

 and places under the influence of preceding division planes; but their 

 normal occurrence may be interfered with by an abnormal chemical condi- 

 tion of the environment. 



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