22 ARTIFICIAL PARTHENOGENESIS AND FERTILIZATION 



sis of nucleins from lecithin, I have indicated that cholin must be 

 set free in the hydrolysis of lecithin. 1 Robertson assumes that 

 this cholin, which is an alkali, may be the substance which serves 

 to form the soap, and hence induces cell division. 2 Obviously 

 other alkalies may also be concerned. Perhaps in plants an 

 acid substance cellulose is conveyed to the equatorial plane, 

 and hence in this case a separation of the two cells does not take 

 place, but only the formation of a solid separating membrane. 

 This process of cell division is now repeated for each cell. 

 From the two-cell stage (Fig. 13) the egg goes naturally to the 

 four-cell stage (Fig. 14), eight-cell stage (Fig. 15), etc. 



FIG. 14. Four-cell stage of 

 the egg of S. purpuratus. 



FIG. 15. Eight-cell stage of the 

 egg. 



From the early stages there is a marked tendency for the 

 single cells to creep to the surface of the egg; this may depend 

 upon a tropism, perhaps a positive chemotropism of the cells 

 toward oxygen. Owing to this creeping of the cells to the 

 surface the first larval stage of the sea-urchin is a hollow sphere, 

 the so-called blastula (Fig. 16). In this stage there appear on 

 the outer surface of the cells cilia (which are omitted in Fig. 16) 



1 Loeb, Ueber den chemischen Charakter des Befruchtungsvorgangs, und seine 

 Bedeutung fur die Theorie der Lebenserscheinungen, Leipzig, 1908. 



2 The interpretation of Robertson's experiment was combated by McClendon, 

 Am. Jour. Physiol., XXVII, 240, 1910, and Archiv f. Entwicklungsmechanik, 



XXXIV, 263, 1912. See also T. B. Robertson, Archiv f. Entwicklungsmechanik, 



XXXV, 692, 1913. 



