September 14, 1900.] 



SCIENCE. 



391 



The layer of cells which contributes, both 

 as regards the number and variety of the 

 tissues derived from it, most largely to the 

 formation of the body is the middle layer 

 or mesoblast. From it the skeleton, the 

 muscles, and other locomotor organs, the 

 true skin, the vascular system, including 

 the blood, and other structures which I 

 need not detail, take their rise. From the 

 inner layer of cells or hypoblast, the prin- 

 cipal derivatives are the epithelial lining of 

 the alimentary canal and of the glands 

 which open into it, and the epithelial lin- 

 ing of the air-passages. The outer or epi- 

 blast layer of cells gives origin to the epi- 

 dermis or scarf skin and to the nervous 

 system. It is interesting to note that from 

 the same layer of the embryo arise parts so 

 different in importance as the cuticle — a 

 mere protecting structure, which is con- 

 stantly being shed when the skin is sub- 

 jected to the friction of a towel or the 

 clothes — and the nervous system, including 

 the brain, the most highly differentiated 

 system in the animal body. How com- 

 pletely the cells from which they are de- 

 rived had diverged from each other in the 

 course of their differentiation in structure 

 and properties is shown by the fact that 

 the cells of the epidermis are continually 

 engaged in reproducing new cells to replace 

 those which are shed, whilst the cells of 

 the nervous system have apparently lost 

 the power of reproducing their kind. 



In the early stage of the development of 

 the egg, the cells in a given layer resemble 

 each other in form, and, as far as can be 

 judged from their appearance, are alike in 

 structure and properties. As the develop- 

 ment proceeds, the cells begin to show dif- 

 ferences in character, and in the course of 

 time the tissues which arise in each layer 

 differentiate from each other and can be 

 readily recognized by the observer. To use 

 the language of von Baer, a generalized 

 structure has become specialized, and each 



of the special tissues produced exhibits 

 its own structure and properties. These 

 changes are coincident with a rapid multi- 

 plication of the cells by cleavage, and thus 

 increase in size of the embryo accompanies 

 specialization of structure. As the process 

 continues, the embryo gradually assumes 

 the shape characteristic of the species to 

 which its parents belonged, until at length 

 it is iit to be born and to assume a separate 

 existence. 



The conversion of cells, at iirst uniform 

 in character, into tissues of a diverse kind 

 is due to forces inherent in the cells in each 

 layer. The cell plasm plays an active 

 though not an exclusive part in the special- 

 ization ; for as the nucleus influences nu- 

 trition and secretion, it acts as a factor in 

 the differentiation of the tissues. When 

 tissues so diverse in character as muscular 

 fiber, cartilage, fibrous tissues, and bone 

 arise from the cells of the middle or meso- 

 blast layer, it is obvious that, in addition to 

 the morphological differentiation affecting 

 form and structure, a chemical differen- 

 tiation affecting composition also occurs, as 

 the result of which a physiological differ- 

 entiation takes place. The tissues and 

 organs become fitted to transform the energy 

 derived from the food into muscular energy, 

 nerve energy, and other forms of vital ac- 

 tivity. Corresponding differentiations also 

 modify the cells of the outer and inner 

 layers. Hence the study of the develop- 

 ment of the generalized cell layers in the 

 young embryo enables us to realize how all 

 the complex constituent parts of the body 

 in the higher animals and in man are 

 evolved by the process of differentiation 

 from a simple nucleated cell — the fertilized 

 ovum. A knowledge of the cell and of its 

 life-history is therefore the foundation- 

 stone on which biological science in all its 

 departments is based. 



If we are to understand by an organ in 

 the biological sense a complex body capable 



