FERTILIZATION AND CLEAVAGE IO7 



In centrifuged whole eggs, after fertilization, the micromeres may come 

 off at any place, even at the oil cap (E. B, H.). Their position is 

 probably determined by the original polarity of the egg. They may be 

 pigmented but are usually not. See Plate VIII, Photographs 5, 6,15, 1 6. 

 There may be not four micromeres, but three, two, one or none, 

 and yet normal development may follow. This has been found by 

 Tennent, Taylor, and Whitaker (1929) and also by Horstadius (1937 a). 

 Horstadius also calls attention to the fact that not all small cells are 

 micromeres, but that small cells may be formed as the result of entirely 

 different factors from those leading to micromere formation. These 

 facts must be born in mind in any investigations involving micromeres. 



n. Absence of Oxygen 



Whether an egg can be fertilized in absence of oxygen has always been 

 an intriguing question. Unfortunately for its answer, oxygen is neces- 

 sary for motility of the sperm, so that in its absence they cannot swim 

 to the eggs and fertilize them. If there is the slightest trace of oxygen 

 so that a few sperm are very slightly motile, a fertilization membrane 

 is thrown off, but no further development takes place (E. B. Harvey, 

 1930). That it is the lack of motility of the sperm and not the absence 

 of oxygen is shown by an experiment of Kitching and Moser (1940). 

 Arbacia eggs were kept in absence of oxygen beside a drop of a parthe- 

 nogenetic agent, and then the two drops were mixed in absence of 

 oxygen. A fertilization membrane was formed. 



o. Refertilization 



It has been the experience of many investigators that an tgg, once 

 fertilized, cannot be fertilized again, even if the fertilization membrane 

 has been removed (Loeb, 1 9 1 6, p. 85 ; F. R. Lillie, 1 9 1 9, p. i&i \ et al.). 

 Until recently an only exception for sea urchins is the report of Bury 

 (19 1 3) that if the eggs of Strongylocentrotus lividus and Echinus micro- 

 tuberculatus, already fertilized and with fertilization membranes, are 

 kept in the cold, 0° C, they can be refertilized. Recently, Sugiyama 

 ("1947, preliminary; 1951) has found that in Strongylocentrotus pulcher- 

 rimus and other Japanese species, after the fertilization membranes of 

 fertilized eggs have been mechanically removed, and the eggs are 

 washed in Ca-Mg-free sea water, they could be refertilized. The sperm 

 penetrated the eggs and took part in the formation of the mitotic 

 figures producing irregular cleavages characteristic of polyspermy; the 



