No. 3-] XER/'l'LOKBIX SQUAMIGERUS CARPENTER. 119 



form the tip cells of the arms of the cross. The four tip cells 

 are smaller than the others, as in Crepidula and Planorbis. A 

 cleavage of the four lower cells -of the second quartette has 

 taken place, likewise in a laeotropic direction. The second 

 quartette now contains sixteen cells in four groups of four cells 

 each. These cells bear exactly the same relations to each 

 other, to the arms of the cross, and to the adjacent cells of the 

 other quartettes, that they do in Crepidula, Planorbis, and 

 several other forms at the corresponding stage of development. 

 There can be no doubt, therefore, of their derivation, though 

 their actual divisions were not all observed. The large ento- 

 meres, A, B, and C, have divided laeotropically, completing the 

 formation of the fourth quartette. In place of the mesoblast 

 cell ^D there is a group of four cells, an upper pair containing 

 little yolk, and a lower pair of about the same size in which the 

 yolk is abundant. The origin of these cells was not followed, 

 but there can be little doubt that they all owe their origin to 

 the mesoblastic pole cell. They occupy the same area that 

 was occupied by the pole cell. The cap of ectodermic cells is 

 radially symmetrical, and contains no cells of sufficient size to 

 have given rise to such large cells as the upper pair of the four 

 without altering very materially the symmetrical relations shown 

 in the figure. Besides, nothing corresponding to such a division 

 is seen in other forms. In all probability these cells arose first 

 by a horizontal division of the mesoblast cell, such as occurs in 

 a large number of forms, and then by a division of the two 

 daughter-cells in a plane at right angles to the previous one. 

 At this period the egg contains a regular cap of ectodermic 

 cells, four entomeres at the vegetal pole, three entomeres, 4 a, 

 4 b, and 4 c of the fourth quartette, and the group of four cells 

 above described, in place of the remaining cell of the fourth 

 quartette, 4 d. A comparison with the corresponding stage in 

 the egg of Crepidula, as shown in Fig. 31 of Conklin's paper, 1 

 shows that the cells of the ectodermic cap correspond point 

 for point, and also that the four cells we have derived from qd 

 are represented in Crepidula by four cells of subeqnal size having 

 the same origin and position. The fourth quartette is formed a 



1 Jcnrn. Morph. Vol. xiii. 1897. 



