22 ARTIFICIAL PARTHENOGENESIS AND FERTILIZATION 
sis of nucleins from lecithin, I have indicated that cholin must be 
set free in the hydrolysis of lecithin.! 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), ete. 
Fie. 14.—Four-cell stage of Fie. 15.—Eight-celi stage of the 
the egg of S. purpuratus. 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 fiir die Theorie der Lebenserscheinungen, Leipzig, 1908. 
2 The interpretation of Robertson's experiment was combated by McClendon, 
Am. Jour. Physiol., X XVII, 240, 1910, and Archiv f. Entwicklungsmechanik, 
XXXIV, 263, 1912. See also T. B. Robertson, Archiv f. Entwicklungsmechanik, 
XXXV, 692, 1913. 
