248 



EVOLUTION AND ANIMAL LIFE 



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not simply divide and t^us make two masses each capable of 

 the growth and change necessary to make it like the parent 

 mass, but where the parent mass (a fertilized egg cell, or a sexual 

 egg or bud cell) can grow and develop into a highly complex 

 many-celled new organism of type like that from which the 

 parent germ plasm was derived. The special capacities, there- 

 fore, of germ plasm have furnished for centuries, and do to-day, 



the great problem of biology 

 (next to that provided by the 

 existence of life itself). 



If we cling to a belief that 

 in some way, after all, the ex- 

 planation of the general proto- 

 plasmic and special germ plasm 

 capacities lies in an unusual 

 combination of structure and 

 play of familiar form of energy 

 through the structure, we arc 

 at once forced to assume a 

 structural make-up of proto- 

 plasm and germ plasm beyond 

 the highest powers of our mi- 

 croscopes to detect. And this 

 assumption actually is made 

 by most biologists. No agree- 

 ment, however, exists among biologists as to this assumed 

 structure. Biology does not have its atomic theory as chemistry 

 does, to explain the ultramicroscopic make-up of the sub- 

 stances with which it has to deal, but has its atomic theories, 

 a score of fairly well-marked theories as to the ultimate struc- 

 ture of germ plasm having been advanced in the last couple of 

 centuries of biologic study. 



Almost all of these theories assume a micromeric structure 

 of protoplasm; a few are antimicromeric. By mjcromerio is 

 meant simply that the plasm which appears to us as a viscous 

 colloidal substance, somewhat differentiated into denser and 

 less dense parts, appearing as fibrils or grains or alveoles in a 

 ground substance of different density, is assumed to be com- 

 posed of myriads of minute, ultramicroscopic units of the 

 general nature of combinations of chemical molecules. These 

 unit combinations are given^ in the theories of various authors, 



Fig. 144. — Egg cell of a sea urchin, Toxo- 

 pneustes lividus, showing cytoplasm, 

 nucleus, and nucleolus, and network 

 or alveolar appearance of the proto- 

 plasm. (After Wilson.) 



