ADDRESS BEFORE THE BRITISH ASSOCIATION. 4< 



pecially in the higher animals, and the important conclusion has been 

 arrived at that each kind of tissue invariably arises from one of these 

 layers and from no other. 



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 the principal derivatives are 

 the epithelial linings of the alimentary canal and of the air passages. 

 The outer layer of cells gives origin to the epidermis 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 constantly being shed when the skin 

 is subjected 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 completely 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 appar- 

 ently 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 devel- 

 opment proceeds, the cells begin to show differences 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 multiplication 

 of the cells by cleavage, and thus increase in size of the embryo ac- 

 companies 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 fit to be born and to 

 assume a separate existence. 



The conversion of cells, at first 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 nutrition 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 mesoblast layer, it is obvious that, in 

 addition to the morphological differentiation affecting form and struc- 



