6o BIOLOGICAL LECTURES. 



necessary was that one part of the protoplasm should flow out. 

 The chief facts for causal morphology are the chemical differ- 

 ences and the local distribution of the chemically different 

 material by molecular forces. 



In regard to the question why under normal conditions 

 the entoderm is pushed inside the segmentation -cavity 

 I wish to direct attention to a point that, as far as I 

 know, has not yet been considered. The sea-urchin's egg, 

 during the first stages of development, has a higher specific 

 gravity than the sea-water. In the blastula stage, however, 

 its specific gravity decreases and the egg begins to float and 

 comes at last to the surface of the water. This is possible 

 only through the eggs taking up substances which have a 

 lower specific gravity than the sea-water, and the only sub- 

 stance that could, in this case, be taken up is water. As the 

 cells themselves do not become larger, we must assume that 

 by a process of secretion the segmentation-cavity is filled with 

 a liquid of lower specific gravity than sea-water. In this way 

 the cells of the blastula come to have their peripheral surfaces 

 in contact with sea-water while their central surfaces are in 

 contact with a fluid containing less salts. This might result 

 in such differences of tension, between the inner and outer 

 walls of the blastula as to determine invagination. I found, 

 last year, that when I raised the eggs in diluted sea-water, 

 evagination (Fig. 28), instead of invagination, took place. 

 Herbst got the same result by using solutions of lithium salts. ^ 

 It is possible, too, that in these cases the hydrostatic pressure 

 within the segmentation-cavity is increased, and this increase 

 of pressure may have a direct or an indirect effect upon 

 growth. 



It is for further investigation, and not for speculation, to 

 settle all these questions in detail. I only wish to point out 

 where, and how far, there exists a relation between substance 

 and form in the early stages of the development of the sea- 

 urchin. 



I have chosen the name Physiological Morphology for these 

 investigations, inasmuch as their object has been to derive the 



1 Ilerbst, Zeitsclir. f. loisscusch. ZooL, Ikl. 55. 



