48 CONKLIN. [Vol. XIII. 



ever, the second cleavage is not yet complete, and can, there- 

 fore, be easily distinguished from the first, and in the ova 

 which are there figured, as well as in hundreds of others which 

 I have studied, the relation of the polar furrows to the first 

 and second cleavages is always the same. 



Similar furrows are shown and described in the works of 

 very many authors, and indeed in the ova of almost every 

 group of animals ; but in most cases no mention is made of 

 any definite relation between these furrows and the first and 

 second cleavage planes. In the frog, according to Rauber 

 ('82), this furrow bears no constant relation to the first two 

 cleavages, and Eycleshymer ('95) seems to have found the 

 same thing true of Ambly stoma, Petromyzon, and Corregonus. 

 But in a very large number of animals, belonging to groups 

 as far removed from each other as mollusks, annelids, and 

 polyclades, the relation between the polar furrow and the first 

 and second cleavages is a constant one. In Blochmann's 

 figures of the egg of Neritina, and in Lang's figures of Dis- 

 cocoelis, the polar furrow is shown bending to the right^ in the 

 first cleavage (the position which it has in Crepidula), though 

 neither of these investigators calls attention to this fact in the 

 text or description of figures.^ The same fact is further shown 

 and commented on by Wilson ('92) in the case of Nereis, 

 Heymons ('93) in Umbrella, and Lillie ('95) in Unio. A very 

 striking exception to this rule has been discovered by Crampton 

 ('94) in the case of Physa, a sinistral gasteropod, in which 

 the direction of the polar furrow is reversed, and he points 

 out the fact that the figures which Rabl ('79) gives for Planorbis, 

 and a figure given by Haddon ('82) for Janthina, seem to show 

 a similar reversal. So far as I know these are the only cases 

 on record in which the polar furrow constantly turns to the 

 left when seen in the first furrow, wher^s in many cases, as 

 I have indicated, it constantly turns to the right.^ 



1 One figure which Blochmann gives, Fig. 40, corresponds very closely with my 

 Figs. 9 and 10 ; the second furrow is still incomplete, and two of the macromeres 

 are much more obtuse at the centre than the other two. The polar furrow thus 

 formed bends to the right in the first furrow just as it does in Crepidula. 



- Since this was written Kofoid's final paper on Lima.x ('95) has appeared, in 

 which he thoroughly discusses the "cross furrows," especially the relation of the 



