THE INDIVIDUAL AND THE SPECIES 177 



nuclear part of the cell — the cytoplasm — is modified 

 in various ways : thus it may possess flagella, or 

 cilia, so that it may be actively locomotory. It is at 

 once a receptor apparatus, susceptible to changes in 

 the medium in which it lives, and it is also an effector 

 apparatus, capable of transforming stimuli received into 

 motor impulses. It may be able to accumulate avail- 

 able energy by making use of the energy of radiation 

 in the synthesis of carbohydrate and proteid from the 

 inorganic substances in solution in the water in which 

 it lives ; and it is also able to expend this energy in 

 controlled movements. All the characteristics of life, 

 in fact, are exhibited by the unicellular organism, 

 the differentiation of the cytoplasm corresponding 

 functionally to the differentiation of the tissues of the 

 multicellular animal or plant. 



In the latter the organs, organ-systems, and tissues 

 are composed of differentiated cells. Development 

 consists essentially of a process of cell-formation by 

 simple division, and at the end of this process of 

 segmentation various rudiments (Anlagen) are estab- 

 lished. The older embryologists sought to recognise 

 the formation of three " germ-layers " in most groups 

 of animals : these were the outer layer or ectoderm, 

 the middle layer or mesoderm, and the internal layer 

 or endoderm. The ectoderm, it was held, gave rise 

 to the integument, the central and peripheral nervous 

 systems, and the sensory organs. The mesoderm gave 

 rise to the musculature and skeleton, the excretory 

 organs, and some other tissues. The endoderm gave 

 rise mainly to the alimentary canal and its glands. 

 The " Gastrea-Theory " of Haeckel sought to recognise 

 a similar larval form, or " Gastrea," in the develop- 

 ment of most multicellular animals, and much 

 ingenuity of argument was required for the estab- 



M 



