248 



EVOLUTION AND ANIMAL LIFE 



not simply divide and thus 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 are 

 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 micromeric 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.) 



