Febeuaky 16, 1900.] 



SCIENCE 



249 



usually in the form of reversions to one or 

 the other of the parent species. The possi- 

 bility presents itself then, that this rever- 

 sion may be due to the persistence of the 

 chromatin of only one species in one or both 

 of the germ cells. Carrying the conception 

 still further, the other variations in the off- 

 spring of hybrids may be due, perhaps, to 

 the varying proportions of the chromatin of 

 each species in the mature germ cells. 



The egg of the Stichostemma. C M. Child. 

 During the autumn of 1899 a species of 

 Stichostemma, probably S. asensoriatum Mont- 

 gomery, was found in great abundance in a 

 park lagoon in Chicago. In this small 

 fresh-water nemertean, the gonads occupy 

 the position usual in nemerteans, i. e., be- 

 tween the diverticula of the intestine. 

 The animal is hermaphroditic and probably 

 more or less completely protandric, though 

 spermatozoa are often found in the same 

 gonads with ripe eggs. The gonad is in the 

 form of a follicle, which is lined with the 

 germinal sj'ncytium. The portion of the 

 syncytium which is to form the egg, contain- 

 ing one nucleus, bulges out into the cavity 

 of the follicle and grows in size so that the 

 follicle is filled. It remains attached to the 

 remainder of the germinal mass by a pedicle, 

 and yolk spheres are formed not only 

 within the limits of the oocyte itself but 

 also in the protoplasm about the pedicle. 

 As the oocyte grows, the greater portion of 

 the germinal protoplasm appears to pass 

 into it and all or nearly all of the germinal 

 nuclei disappear leaving a single oocyte in 

 the gonad. Sometimes two oocytes instead 

 of one are formed in one gonad. The 

 growing oocyte forms a membrane about 

 itself, but the stalk remains until the egg is 

 laid, when it is broken off and the mem- 

 brane, which swells rapidly under the ac- 

 tion of water, closes over this portion of the 

 cell also. Thus the oocyte does not become 

 a distinct cell until the moment of laying. 



The pore of the gonad is apparently not 

 preformed, appearing at the time of laying. 

 As it is very small the egg is greatly de- 

 formed in its passage, but its protoplasm is 

 so fluid that it flows out easily, in a fine 

 stream, all except the nucleus, which often 

 appears to block the way for a moment and 

 then pops out suddenly. After laying the 

 egg soon assumes a spherical form and the 

 membrane swells to a considerable thick- 

 ness. Eggs are laid normally in strings 

 somewhat less in length than the body of 

 the worm, and containing two rows of eggs 

 in a considerable amount of slime secreted 

 by the skin, and attached to plants, etc. 



Self-fertilization may occur (probably 

 occurs normally in many cases) and is fol- 

 lowed by normal development. Almost 

 immediately after fertilization the whole 

 surface of the egg becomes roughened in 

 consequence of ectoplasmic activity, and 

 soon accumulations of liquid appear be- 

 tween the egg and the membrane. Various 

 changes of form occur during the formation 

 of the two polar bodies, and, the vitelline 

 membrane being absent, amceboid proc- 

 esses of the ectoplasm appear at many 

 points. Soon after their formation the 

 polar bodies begin to enlarge and become 

 transparent, and before the first cleavage 

 is completed they have usually disappeared. 

 In the manner of their degeneration they 

 resemble very closely injured or unripe eggs 

 which die in the water. These increase in 

 size very rapidly and become vacuolated 

 and soon the yolk becomes liquid and dis- 

 appears. 



Just before and during the first cleavage 

 pseudopodia-like outgrowths of the ecto- 

 plasm are abundant in the region of the 

 cleavage-furrow. The ectoplasm is clearly 

 continuous between the cells. 



In the resting two-cell stage, again in the 

 four-cell stage, and between adjacent cells 

 in subsequent stages temporary cavities ap- 

 pear which are filled with liquid. These 



