76 THE AMERICAN ARBACIA 



perfectly until several hours after the second polar body has been 

 given off. 



\{ Arbacia eggs are centrifuged at the time of polar body formation, 

 it has been found that the polar bodies may come off in any relation 

 to the stratification, in the yolk or pigment zone or even at the oil 

 cap (E. B, H.) (Photographs 14 and 15). It seems that the original 

 polarity of the egg must determine their position and not the polarity 

 imposed by centrifugal force expressed by the stratification. 



Usually the polar bodies of Arbacia have been thrown off and dis- 

 carded by the time the eggs are laid or taken from the ovary, but in 

 some batches they are retained in the majority of the eggs. In such a 

 batch, Hoadley (1934) found that they may lie in any position with 

 regard to the (mature) nucleus, and I have confirmed this observation 

 in other batches. The polar bodies in Paracentrotus lividus are given off 

 in the micropyle (Boveri, 1901), and also in Lytechinus (Tennent, 

 Taylor, and Whitaker, 1929); this is true also of Arbacia. The first 

 cleavage plane passes through the region of" the polar bodies. The 

 micromeres are formed at the vegetal pole, opposite the polar bodies. 

 Previous observers also found that the micromeres formed "approxi- 

 mately opposite to the micropyle" in Arbacia (Morgan and Spooner, 

 1909, p. 116; Spooner, 191 1; Horstadius, 1937a). 



Other Species (Additional) and General References 



A. Brachet, 1922. Paracentrotus lividus, fertilization of immature eggs. 



J. Brachet, 1933. Paracentrotus lividus, Feulgen stain. 



Bryce, 1902. Polar bodies, sections. 



Derbes, 1847. "Oursin comestible"; earliest figure of immature egg. 



Harris, 1939. Echinus esculcn'.us, viscosity and polarity; fall of nucleolus. 



Lindahl and Holter, 1941. Paracentrotus lividus, respiration. 



Lyon and Shackell, 1910 b. Toxopneustes variegatus, permeability. 



Runnstrom, 1928c. Paracentrotus lividus, etc., papillae, surface. 



Selenka, 1878. Toxopneustes variegatus, early description. 



Skowron and Skowron, 1926. Sphaerechinus granularis, permeability. 



Tennent, Gardiner, and Smith, 1931. Echinometra lucunter, stains for micro-chemistry. 



von Ubisch, 1950. Paracentrotus lividus, general. 



Wilson, 1895. Atlas of Fertilization. 



Wilson, 1899. Protoplasmic structure. 



Wilson, 1925. The Cell, General. 



Wilson and Mathews, 1895. Toxopneustes variegatus, polarity. 



