194 DYNAMICS OF LIVING MATTER 



In these deformed eggs the distribution of the nuclear material 

 during cell division is entirely different from that which normally occurs ; 

 yet normal embryos result. Driesch has shown the same in a different 

 way; namely, by submitting the developing eggs to pressure. In 

 eggs thus flattened, the planes of segmentation differ from those of the 

 normal egg, yet normal embryos are formed. These observations 

 exclude the idea that the distribution of the nuclear material through 

 the egg is of importance for the form of the embryo. 



Driesch succeeded in causing fertilized sea urchins' eggs to fuse in 

 a number of cases. Such a fusion of the masses of two fertilized eggs 



into one, resulted in the formation of a sin- 

 gle giant embryo (pluteus).* Such a result 

 would be inconceivable did the egg possess 

 a structure of such a degree of complexity 

 as the adult animal. Zur Strassen \ had 

 already before Driesch's experiments made 

 the observation that the eggs of a parasitic 

 FIG. 38. worm, Ascaris, occasionally give rise to 



giant embryos through the fact that two 



eggs fuse and that their combined masses now give rise to but one 

 organism. If the egg possessed a complicated structure, the fusion 

 of the masses of two eggs could no more give rise to a single individual 

 of gigantic dimensions than two individual adult animals could be 

 transformed into one by fusing their masses. I have also observed 

 that with the proper chemical treatment the eggs of the starfish and 

 of Ch&topterus can be caused to fuse; that from two or more such 

 eggs a single giant embryo may result.^ 



Boveri and Driesch assume the existence of a certain simple struc- 

 ture in the unfertilized egg of the sea urchin. According to Boveri, 

 the egg protoplasm consists of three layers occupying different part 5 

 of the egg (see Fig. 6, p. 31). These three masses can still be recog- 

 nized in the first four cleavage cells, but in the eight-cell stage cells 

 arise which no longer contain all three layers. It is possible that only 

 such isolated cells can give rise to a single embryo, as contain all three 

 layers. This may account for the fact that an isolated blastomere of 

 the four-cell stage can still develop into a normal embryo, while the 

 same is no longer true for the isolated cell of the eight-cell stage. 

 As far as the possible origin of the differentiation observed by Boveri 

 is concerned, I have noticed in an Ophiurian that the immature eggs 



* Driesch, Archiv fur Entwickelungsinechanik, Vol. IO, p. 411, 1900. 

 f Zur Strassen, Archiv fur Entwickelungsmechanik, Vol. 7, 1898. 

 J Loeh, Am. Jour. Physiology, Vol. 4, p. 423, 1901. 

 See Lecture 2. 



