Effects of Environment 343 



structure of segmenting and multiplying cells reaches the 

 stage of the gastrula, which is the spherical bell with an 

 interior cavity; the full qualification of the life-plasm re- 

 mains only in a few cells, the others having lost some of the 

 abilities, and become specialized for certain duties. The fully 

 qualified cells continue to divide, and they produce germ cells, 

 which are in fact part and continuation of the original life- 

 plasm. Now some of these gastrulae allow the germ cells 

 to issue like buds upon the outside of the bell, while others 

 adopt the habit of budding them upon the inside surface. 

 So long as all this life remained aquatic (as all life was 

 originally) the differences of these two habits would be 

 chiefly in the protection to offspring afforded by the inward 

 budding, and missed by the outward. While on the other 

 hand the outward budding creatures would enjoy a method 

 of colony growth, by which the bud could remain attached ; 

 while the inward produced buds must obviously detach them- 

 selves. And so both systems prevailed and founded many 

 species. But later, when organized creatures began to leave 

 the waters or were left by them a new difference appeared; 

 as may be studied at this day, among animals of these early 

 types. Those of external buds could not carry on their 

 process of budding except by remaining immersed, because 

 the heredity of their germ-plasm could not be developed 

 except in its aquatic environment. But those of internal buds, 

 by the simple device of closing the cavity could retain the 

 needed fluid medium and preserve the bud during periods 

 of land exposure. So it became possible for them to be 

 amphibious and to return to the water occasionally and there 

 to deposit eggs which had been internally fertilized and de- 



