78 EDWARD DRANE CRABB. 



Moquin-Tandon ('55) was unable to find a vitelline membrane but 

 does not indicate the species of snail he was studying. Other 

 forms have been described as not possessing a definite vitelline 

 membrane ; thus it is not surprising to find this membrane want- 

 ing in the eggs of Lymncea. 



VII. MATURATION OF VIRGIN EGGS. 



Since in the maturation of eggs laid by snails which were iso- 

 lated in ovo no marked differences from several published de- 

 scriptions of supposedly cross-fertilized eggs of this and other 

 forms of Lymncea, as well as those of pond snails in general 

 (Conklin, '10; Gatenby, '19; Kostanecki, '97; Morgan, '10, and 

 others) were found, most of my evidence will be directed to 

 points of comparison with maturation in forms other than L. s. 

 appressa. I have observed two maturation divisions in living 

 virgin eggs of L. s. appressa,, L. palustris and PJiysa gyrina, and 

 Colton ('18) found that L. columclla regularly gives off two polar 

 bodies. On the other hand Pelseneer ('19, p. 1058) is con- 

 fident that virgin eggs of L. auricularis and L. glutinosus as well 

 as L. palustris give off only a single polar body which never 

 divides and is visible up to the gastrula stage. I am certain that 

 Pelseneer's observations are incorrect in this respect, for in com- 

 paring virgin with normal eggs in the genus Limncea he makes 

 no mention of having ever found the first polar body isolated in 

 the albumen of the egg. I have found the first polar body com- 

 pletely separated from the vitellus in L. palustris while in L. ^. 

 appressa it generally migrates into the albumen frequently to a 

 distance of 50 to 200 micra or more before the first cleavage fur- 

 row is visible (Figs. 34, 35), consequently it may be easily over- 

 looked in either living eggs or in sections. Byrnes ('99, p. 207) 

 found that in normal Limax agrestis the first polocyte separates 

 from the egg and collapses. In general Pelseneer's description 

 is that of the second polar body, therefore I believe that he mistook 

 the second for the first polar body. 



In most instances the first polocyte is given off in from two to 

 two and one half hours after the eggs have been oviposited. 

 However, exceptions to the rule are numerous in L. 5. appressa. 

 In several instances the mass of eggs was seized with forceps be- 



