50 BIOLOGICAL LECTURES. 



Driesch has shown l that by shaking a sea-urchin's egg in the 

 four-celled stage the four cells may be separated, and each one 

 be capable of giving rise to a complete embryo, which is only 

 different in size from the normal enrbryo. If the theory of 

 preformed germ-regions with its later modifications were true, 

 we ought to expect that every one of the isolated cells would 

 give rise to one-fourth of an embryo. But it has been said 

 that the artificial isolation of one cleavage cell causes a process 

 of post generation or regeneration. Driesch, moreover, changed 

 the mode of the first cleavage by submitting the ovum to one- 

 sided pressure. In this way the nuclei were brought into 

 somewhat different places from those they would have held in 

 the case of normal segmentation. Still, normal embryos 

 resulted. One might object again that the preformation of 

 the germ-regions existed in the protoplasm, and not in the 

 nucleus. I have made a series of experiments to the results 

 of which these objections cannot be made. I shall describe 

 these experiments somewhat fully, as they have not yet 

 been published, though I cannot enter into details at this 

 place. 



I brought eggs of a sea-urchin, within ten to twenty minutes 

 after impregnation, into sea-water that had been diluted by 



the addition of about 100 per 

 cent, distilled water. In this 

 solution the eggs took up so 

 much water that the mem- 

 brane (;;/, Fig. 14) burst and 

 part of the protoplasm escaped 

 in the form of a drop (b, Fig. 



FIG. 14. vt.-t.-ir 



14), which often, however, re- 

 mained in connection with the protoplasm inside the membrane 

 after the eggs were brought back into normal sea- water. 

 These eggs gave rise to adherent twins, the ejected part of the 

 protoplasm, b, as well as the part remaining inside the mem- 

 brane, developing into a normal and perfectly complete embryo. 

 The part of the protoplasm which at first had connected 

 the two drops, formed the part where the twins remained 



1 Zeitsch riftf. ruissensch. Zoologie, Vol. LIII and LV. 



