I06 THE AMERICAN ARBACIA 



cells which are pigmented. Apparently the pigment recedes from the 

 lower parts of the lower four cells (vegetal) at the eight-cell stage 

 (Morgan, 1893). There has never been a very good explanation for 

 this, nor has any experimental work been done on it. McClendon 

 (1910b, p. 243) thinks that "the pigment entirely disappears from the 

 micromere pole, indicating spreading movements due to the surface 

 tension being less here than in the region of the future cleavage furrow. 

 Similar movements of granules have been observed in the cutting off 

 of polar bodies in various eggs, and it may be concluded that for the 

 separation of a very small cell from a large mass of protoplasm a very 

 great difference in surface tension between the pole of the small cell 

 and the cleavage furrow is required." 



In some cases, it was found by Morgan (1893) that the retreat of 

 pigment to form the micromeres takes place long before the actual 

 cutting off of the cells — in the four or even two-cell stages. An early 

 appearance of micromeres, in the 8-cell stage, was found by Painter 

 (1915) in eggs treated with phenyl urethane. In eggs treated with 

 mustard gas, the micromeres are formed early and one (colorless) 

 micromere is often present in the two-cell stage (E. B. H., 1943 unpub.). 



According to most investigators, the micromeres of Arbacia, as in 

 other sea urchins, come off at the vegetal pole, nearly opposite the 

 funnel in the jelly which marks the position of the polar bodies (Morgan 

 andSpooner, 1909; Spooner, 1911 ;H6rstadius, i937a).Themicromere- 

 forming material was located by Harnley (1926) in the unfertilized 

 Arbacia egg, between the nucleus and the center of the egg. But Ten- 

 nent, Taylor, and Whitaker (1929) could not confirm this, and held 

 that there was no localization of micromere-forming material in the 

 unfertilized eggs, and that in the fertilized egg the micromeres formed 

 at the cut surface of any fragment. Horstadius (1937a) could not 

 confirm this, nor could he confirm Harnley's work. He believes that 

 in Arbacia the micromere-forming material is in the vegetal half of a 

 cut egg, and that the animal half never forms micromeres, the condi- 

 tion he and others have found for other sea urchin eggs. But he admits 

 that "micromere formation is very sensitive to mechanical injury" and 

 "In Arbacia fragments the micromere-formation seems to be inhibited 

 very often" (Horstadius, 1937 a, p. 304). Plough (1927) had, some 

 years before, maintained that there was a localization of skeleton- 

 forming material even before first cleavage in Arbacia eggs as well as 

 in those oi Echinarachnius, Echinus, and Paracentrotus (1927, 1929), just 

 as maintained by Horstadius (1937a). A study of micromeres in egg 

 fragments has been made by Tennent, Taylor, and Whitaker (1929). 



