THE EVOLUTION OF THE INDIVIDUAL. 221 
are nourished by the food-yelk, of which a considerable 
store still remains in the cephalothorax. 
Timagine 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 
suffice 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 bias- 
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 all 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 
