206 



The Mechanistic Conception of Life 



are slightly acid, and at the same time lacking in one of the 

 three important metals, the conditions for the separation of 



Fig. 51 



Fig. 52 



Fig. 53 



Fig. 54 



Figs. 51-54. — Cell-division in a sea-urchin egg, Strongylocentrotus purpura- 

 tus, in normal sea-water. This type of cell-division leads to the formation of one 

 embryo from an egg. M is the fertilization membrane, P a layer of colloidal 

 substance which seems to serve the purpose of keeping all the cells of an egg 

 together. 



the first two cells and the formation of identical twins are 

 provided. 



Fig. 55 



Fig. 56 



Fig. 57 



Fig. 58 



Figs. 55-58. — Cell-division in the egg of Strongylocentrotus purpuratus 

 which leads to the formation of twins. This cell-division can be observed if the 

 egg is put after fertilization into a neutral mixture of salts in which either KCl, 

 or CaClj, or NaCl is lacking. 



In such a neutral solution the substance which forms the elastic layer (PM, 

 Fig. 51) is dissolved. During the segmentation the protoplasm of the egg spreads 

 imtil its long axis touches the fertilization membrane. The two daughter-cells 

 formed (Fig. 57) are separated from each other, instead of remaining connected 

 as in the normal cell-division (Fig. 53). If about one hour later the eggs are put 

 back into normal sea-water each of the two ceUs develops into an embryo (Fig. 58), 

 and the egg thus gives rise to two instead of to one embryo. 



In conclusion it may be pointed out that the reverse result, 

 namely, the fusion of normally double organs, can also be 

 brought about experimentally through a change in the chemical 

 constitution of the sea-water. Stockard succeeded in causing 



