348 PROCEEDINGS OF THE ACADEMY OF [April, 



ing ill eight cells of equal size and occurring at relatively the same time 

 in all four quadrants. 



With the completion of the above-described divisions the large num- 

 ber of cells of similar size at the upper pole of the egg makes their exact 

 lineage difficult to follow, so that it is desirable to make here some com- 

 parisons with the structure and development of the cross and trocho- 

 blasts in other forms, and to bring together the results already obtained 

 before proceeding to more uncertain ground. In formation the cross 

 of Fiona arises in the same manner as in Umbrella and Planorbis, by 

 the completion of the tip cells before the basals ; and in this it differs 

 from Neritina and Crepidula, where the tip arises shortly after division 

 has occurred to form the four basal cells. In Trochus the tips are 

 relatively late in appearing, as the basals have completed their cleavage 

 before these cells arise. At the first cleavage of the basals another 

 striking similarity to Umbrella is found, for in this Opisthobranch the 

 cleavage is Iseotropic, while in Crepidula and Neritina it is dexiotropic, 

 thus breaking the law of alternating cleavages ; and likewise in Planorbis 

 with reversed type the division is Iseotropic and non-alternating with 

 the preceding. Trochus shows an extremely marked Iseotropic division 

 of these cells, so much so, in fact, that the resulting cells he almost 

 transversel}^ In Fiona the anterior and posterior basals are distinctly 

 Iseotropic in origin and so regularly alternating, while considerable varia- 

 tion is found in the lateral arms, a radial type often occurring with Ic^-, 

 sometimes showing a decided dexiotropic direction of spindle. It 

 would appear from this variation in the lateral arms that Fiona shows 

 tendencies toward bilaterality in the first quartet at this time, and such 

 a condition would be in harmony with the l)ilateral cleavages of the 

 third quartet cells, 3c^ and 3d', occurring just previously. However, 

 the radial symmetry of the cross as a whole appears not to be dis- 

 turbed appreciably, so that though these variations may show either a 

 tendency toward bilaterahty or toward entire reversal in all quadrants, 

 as is found in Neritina, Crepidida and Planorbis, this influence has not 

 as yet become sufficiently marked to affect the radial symmetry of 

 the upper pole of the egg to any appreciable degree. In discussing the 

 lack of alternation of these cleavages in Crepidula as opposed to alter- 

 nation in Umbrella, Conklin suggests "upon this difference the future 

 recognizabiUty of the cross in the last-named cases {Crepidula and 

 Neritina) depends". In Umbrella the Iseotropic division of the basals 

 is much more marked than in Fiona, but even in the latter case Conk- 

 lin's prediction is in part, at least, fulfilled, as the cross of Fiona, after 

 a slightly older stage than thus far described, becomes so irregular that 



