170 



THE PHILOSOPHY OF BIOLOGY 



divides, and each of the parts into which it has divided 

 grows to the limiting size, and division again occurs. 

 This is what happens in the case of the growth of the 

 Sea-urchin egg to form the larva, or blastula. The 

 ovum segments into two blast omeres, each of which 

 then grows to a certain extent, and again segments 

 into two blast omeres. After the completion of ten 

 divisions there are about looo cells which are arranged 

 so as to form a hollow ball — the blastula. 



Differentiation is now set up. In the blastula 

 stage all the cells are alike, actually and potentially. 



But soon one part of the 

 hollow ball of cells be- 

 comes pushed inwards, 

 and the cells of this 

 inturned layer become 

 different from those of 

 the external layer, while 

 cells of a third kind 

 appear in the space be- 

 tween the external and 

 internal layers. This is the process of differentiation 

 leading to the development of the various tissues — 

 protective, sensory, digestive, skeletal, etc. The cells 

 still continue to divide and grow to their maximal 

 size, but when the process of differentiation begins, 

 the cells which are formed are not quite the same 

 as those from which they originated. Finally, how- 

 ever, when the rudiments of all the tissues of the 

 adult body have been laid down, the cells begin to 

 produce daughter-cells of only one kind. Growth of 

 the embryo consists, therefore, of the dissociation or 

 division of the substance of the ovum and blastomeres, 

 followed by a gradually increasing differentiation of 

 the cells so produced. 



— Ectoderm 



Mesodermal 

 cells 



-£ndocLerm 



Fig. 20. — The Sea-urchin Gastrula 

 larva in section. 



