THE EVOLUTION OF THE INDIVIDUAL. 221 



are nourished by the food-yelk, of which a considerable 

 store still remains in the cephalothorax, 



I imagine that they are set free during the first ecdysis, 

 and that the appendages of the sixth abdominal somite 

 are at that time expanded, but nothing is definitely known 

 at present of these changes. 



The foregoing sketch of the general nature of the 

 changes which take place in the egg of the crayfish 

 sufiice to show that its development is, in the strictest 

 sense of the word, a process of evolution. The egg is 

 a relatively homogeneous mass of .living protoplasmic 

 matter, containing much nutritive material ; and the 

 development of the crayfish means the gradual conver- 

 sion of this comparatively simple body into an organism 

 of great complexity. The yelk becomes differentiated 

 into formative and nutritive portions. The formative 

 portion is subdivided into histological units : these 

 arrange themselves into a blastodermic vesicle ; the blas- 

 toderm becomes differentiated into epiblast, hypoblast, 

 and mesoblast ; and the simple vesicle assumes the gas- 

 trula condition. The layers of the gastrula shape them- 

 selves into the body of the crayfish and its appendages, 

 while along with this, the cells of which aU the parts 

 are built, become metamorphosed into tissues, each with 

 its characteristic properties. And all these wonderful 

 changes are the necessary consequences of the interaction 



of the molecular forces resident in the substance of the 



16 



