24 REPORT — 1900. 



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 development 

 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 recognised by the observer. To use the language of 

 von Baer, a generalised structure has become specialised, 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 accompanies specialisation 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 specialisation ; 

 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 fibre, 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 structure, a chemical differentiation affect- 

 ing composition also occurs, as the result of which a physiological differen- 

 tiation takes place. Corresponding differentiations also modify the cells 

 of the outer and inner layers. The tissues and organs become fitted to 

 transform the energy derived from the food into muscular energy, nerve 

 energy, and other forms of vital activity. Hence the study of the develop- 

 ment of the generalised cell layers in the young embryo enables us to 

 realise how all the complex constituent parts of the body in the higher 

 animals and in man are evolved by the process of cell growth and differen- 

 tiation from a simple nucleated cell — the fertilised 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 of carrying on a natural process, a nucleated cell is an organ 

 in its simplest form. In a unicellular animal or plant such an organ 

 exists in its most primitive stage. The higher plants and animals again 

 are built up of multitudes of these organs, each of which, whilst having 

 its independent life, is associated with the others, so that the whole may 

 act in unison for a common purpose. As in one of your great factories 

 each spindle is engaged in twisting and winding its own thread, it is at 

 the same time intimately associated with the hundreds of other spindles 

 in its immediate proximity, in the manufacture of the yarn from which 

 the web of cloth is ultimately to be woven. 



It has taken more than fifty years of hard and continuous work to 

 bring our Jinowledge of the structure and development of the tissues and 



