322 



EDWARD D. AND RUBY M. CRABB. 



TABLE II. 



NORMAL AND POLYVITELLINE EGGS LAID BY ABOUT ELEVEN Lymnaa palustris. 



The eggs in group i were laid by two snails kept in one aquarium; 2, eggs from 

 an aquarium containing eight snails; 3, eggs laid by a single snail kept in isolation. 

 The masses in each group are consecutive. 



paratively low number of eggs examined is probably the reason 

 why polyvitelline eggs were not recorded for snails number i F t 5, 

 y, 8 and 10, Table I. In the case of L. palustris the three indi- 

 viduals designated as groups i and 3, Table II., were large adults, 

 but in group 2 only one was a large adult at the time we began 

 examining the eggs. The data relative to the size and age of the 

 twenty-one PJiysa sayil are not sufficient to determine whether 

 or not the size of the individual and number of eggs in the mass 

 are correlated with the presence or absence of polyvitelline eggs. 



DISCUSSION. 



The facts set forth in this paper indicate that polyvitelline eggs 

 may be expected to occur among the normal eggs of old L. s. ap- 

 pressa in the prime of life, but that the laying of these abnormal 

 eggs probably is not a transmissible character. The question 

 whether or not the two or more individuals hatched from the same 

 egg are true twins, triplets, quadruplets, etc., has been raised. 

 The paper of Hall ('25) appears to be the only available record 

 of " twinning " in Mollusca. He found eggs of the tubiculous 

 mollusc Serpuloides verwiicularia containing two embryos which 

 he considered twins. Newman ('23) holds that true twins arise 

 from a single cell and that twinning is " essentially a phenomenon 

 involving a physiological isolation of equivalent parts of the 

 blastoderm and a regulation of the isolated or twinned regions into 

 complete embryos." He also states that he has " never seen a 

 reference to a case of twins or double monstrosity in Mollusca 



