No. 2.] EARLY DEVELOPMENT OF PLANORBLS. 38 7 



the inner ones forming the center, and the cells of the second 

 quartette forming the tips. At its first appearance the cross, 

 therefore, contains twelve cells, eight belonging to the first, 

 and four to the second quartette. From this stage until the 

 period of gastrulation, the cross is a very conspicuous feature of 

 the Qgg. Its further history will be traced in a later section. 



TJie Trochoblasts. — The cleavage of the apical cells of the 

 egg is soon followed by a division of the four outer cells of 

 the first quartette, the trochoblasts, la^, ib^, etc. This division 

 is nearly radial, the upper cell lying in the angles between the 

 arms of the cross directly above the lower one. The two cells 

 are about equal in size and they soon begin to enlarge and 

 become clear. One peculiarity of this cleavage is that it occurs 

 at a much earlier stage than in Umbrella and Crepidula. In 

 the first genus the trochoblasts do not divide until the O-gg 

 contains more than seventy cells ; in the second the anterior 

 trochoblasts do not divide until the number of cells in the Q.gg 

 is over one hundred, while the posterior trochoblasts do not 

 divide until later, if at all. In Unio, however, they divide at 

 about the fifty-cell stage, and at a still earlier stage in Chiton 

 (Metcalf) and Ischnochiton (Heath). If we compare the &gg 

 of Planorbis with that of Limax in this respect, we find a close 

 agreement. The time at which this division takes place in 

 Limax is, according to Kofoid, quite variable, but it occurs, 

 speaking roughly, at about the forty-cell stage. The cells in 

 Limax are comparatively large, as in Planorbis, but, strangely 

 enough, they divide in an entirely different direction. "The 

 axis of the spindle," says Kofoid, "lies parallel to the plane of 

 the equator. There is every indication that the division is 

 nearly meridional (horizontal)." Meisenheimer's Fig. 31 shows 

 horizontal spindles in all four trochoblasts in the ^gg of Limax 

 maximus. The cells resulting from this division lie nearly side 

 by side, instead of the one above the other, as in Planorbis, 

 there being only a slight dexiotropic tendency in the cleavage 

 (see Kofoid, '95, p. 59, Fig. 41). In Conklin's Fig. 50 the cells 

 of the two anterior pairs of trochoblasts lie in nearly the same 

 horizontal plane, and their symmetrical position in relation to 

 the anterior arm of the cross indicates that they were produced 



