396 HOLMES. [Vol. XVI. 



The composition of the cross at this period is as follows 

 (PI. XX, Fig. 37) : Four rather small cells in the center, 

 i^'-'-', i^'"', etc., around the apical pole of the egg; around 

 these are four other cells, the intermediate cells, Irt'■'"^ etc., 

 in the angles between the arms ; the anterior arm, which is 

 broader than the others, consisting of a single basal cell, i^''^'"', 

 then a pair of cells, i(5'"^"'"^"', i^^-''^"'", resulting from the trans- 

 verse division of the inner median cell ; then a single outer 

 median cell, i^'", and finally the tip cell, 2(^'''; the two lateral 

 arms consisting of two pairs of cells at the base, an undivided 

 outer median cell, and a tip cell ; the posterior arm, consisting 

 of a row of four cells. In all, the number of cells in the cross 

 is twenty-nine. The anterior arm is shorter than the others, 

 and the tip cell is more or less clear. The arms of the cross 

 are slightly oblique ; and it is worthy of note that the direction 

 of their inclination is laeotropic, while in Crepidula and Isch- 

 nochiton the arms show a slight dexiotropic twist. This differ- 

 ence is doubtless connected with reverse types of cleavage of 

 these forms. 



The tip cells of the arms have enlarged and become trans- 

 parent. The tip cell of the right arm is v/idest behind, while 

 that of the left is widest in front ; the outer median cells show 

 the reverse relation. The cells of the posterior arm of the 

 cross are usually rhomboidal in outline. The posterior tip cell 

 is large and elongated in the direction of the arm, while the long 

 axis of the other tip cells is transverse to the arm. The ante- 

 rior tip cell is the smallest of the four. As the tip cells in eggs 

 stained with silver nitrate are clear, the portion of the cross 

 which stains dark is that derived entirely from the first quar- 

 tette. This portion is very conspicuous, as it is surrounded 

 on all sides by transparent cells. 



It may be well, before tracing the history of the cross further, 

 to compare it briefly with the cross in other mollusks. Bloch- 

 mann traces the history of the cross in Neritina to a stage in 

 which each arm contained three cells, except the posterior arm, 

 which was composed of four. Blochmann's probably incorrect 

 derivation of the tip cells has already been mentioned, and 

 Kofoid has shown also that it is almost certain that his deri- 



