Reproduction and Development. 55 



as we have seen, was itself developed from the outer layer. 

 But round the retina and the lens there is woven from the 

 middle layer the tough capsule of the eye and the circular 

 curtain or iris. The lining cells of the digestive tube are 

 cells of the inner layer, but the muscular and elastic coats 

 are of middle-layer origin. The lining cells of the salivary 

 glands arise from the outer layer where it is pushed in to 

 form the mouth-pit ; but the supporting framework of the 

 glands is derived from the cells of the middle layer. 



Enough has now been said to give some idea of the 

 manner in which the different tissues and organs of the 

 organism are elaborated by the gradual differentiation of 

 the initially homogeneous ovum. The cells into which the 

 fertilized egg segments are at first all alike; then comes the 

 divergence between those which are pushed in to line the 

 hollow of the cup, and those which form its outer layer. 

 Thereafter follows the differentiation of a special band of 

 outer cells to form the nervous system, and a special rod, 

 derived from the inner cells, to form' the primitive axial 

 support. And when the middle layer has come into exist- 

 ence, its cells group themselves and differentiate along 

 special lines to form gristle or bone, blood or muscle. 



The description above given is a very generalized and 

 diagrammatic description. There are various ways in 

 which complexity is introduced into the developmental 

 process. The store of nutritive material present in the 

 egg, for example, profoundly modifies the segmentation 

 so that where, as in the case of birds' eggs, there is a large 

 amount of food-yolk, not all the ovum, but only a little 

 patch on its surface, undergoes segmentation. In this little 

 patch the embryo is formed. Break open an egg upon 

 which a hen has been sitting for five or six days, and you 

 will see the little embryo chick lying on the surface of the 

 yolk. The large mass of yolk to which it is attached is 

 simply a store of food-material from which the growing 

 chick may draw its supplies. 



For it is clear that the growing and developing embryo 

 must obtain, in some way and from some source, the food- 



