ADDRESS. 23 



from pre-existing living matter, even when it possesses the simplicity of 

 structure of a bacterium, and the morphological unit is the cell. 



Development of the Egg. 



As the future of the entire organism lies in the fertilised egg cell, we 

 may now briefly review the arrangements, consequent on the process of 

 segmentation, which lead to the formation, let us say in the egg of a 

 bird, of the embryo or young chick. 



In the latter part of the last century, C. F. Wolff observed that the 

 beginning of the embryo was associated with the formation of layers, and 

 in 1817 Pander demonstrated that in the hen's egg at first one layer, 

 called mucous, appeared, then a second or serous layer, to be followed by a 

 third, intermediate or vascular layer. In 1828 von Baer amplified our 

 knowledge in his famous treatise, which from its grasp of the subject 

 created a new epoch in the science of embryology. It was not, however, 

 until the discovery by Schwann of cells as constant factors in the struc- 

 ture of animals and in their relation to development that the true nature 

 of these layers was determined. "We now know that each layer consists 

 of cells, and that all the tissues and organs of the body are derived from 

 them. Numerous observers have devoted themselves for many years to 

 the study of each layer, with the view of determining the share which it 

 takes in the formation of the constituent parts of the body, more especially 

 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 or hypoblast, the principal derivatives are the 

 epithelial lining of the alimentary canal and of the glands which open into 

 it, and the epithelial lining of the air-passages. The outer or epiblast layer 

 of cells gives origin both 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 derived 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 systeip have apparently lost the power of reproducing 

 their kind. 



