528 PROCEEDINGS OF THE ACADEMY OF [I)eC,„ 



First quartet 16 cells. 



Second quartet, 8 " 



Third quartet 4 " 



Basal quartet, 4 " 



32 " 



From the above it is noticeable that the cells of the first (juartet 

 show a tendency towards rapid division. In a number of moUusks^ 

 e.g., Neritvna (Blochman, 81), Unio (Lillie, 95), Crepidvla (Conklin, 97) 

 and Fiona (Casteel, 04), the first quartet has divided only once before 

 the third quartet is formed. In Umbrella (Heymens, 93) and Urosal- 

 pinx (Conklin, 91) the first quartet does not divide at all until after the 

 third is formed. On the other hand, in the polyclads so far studied 

 (Lang, 84), in N cries (Wilson, 92), Liniaz (Kofoid, 94), and in 

 Dinophilus (Nelson, 04), the first quartet divides twice before or at the 

 time the third is forming. As Conklin (97) has pointed out, this indicates 

 the general rate of development of the upper hemisphere. In Plano- 

 cera this is exceedingly rapid as its further history will show. 



Formation of the Fourth Quartet — Thirty-two to Forty Cells. 



After the thirty-two-cell stage the divisions become more or less 

 irregular; certain cells divide several times before others divide at alL 

 After the completion of the thirty-two-cell stage the next cells to 

 divide are the large basal quartet cells, 3D and 3/1 The nuclei of these 

 cells have moved from the upper to the lower edge of cells, and when 

 the spindle forms it reaches from the centre of the cell downward to its 

 lower margin (figs. 15 and 16). These spindles are very nearly radial 

 in position. Concerning them in Discocoelis Lang says (p. 335): "Nur 

 sehr schwach ist die Abschniirung in der Richtung einer rechts gewun- 

 denen Spirale (wenn wir das Ei so orientiren, dass der orale Pol unten, 

 der aborale oben liegt, und der Beobachter in der Achse dcs Eies steht) 

 angedeutet." By comparing figs. 20 (PI. XXW'II). 25 and 26 (11. 

 XXXVIII) and 30 (PL XXXIX), it will be seen that the small macro- 

 meres show a twisting towards the left as seen from below, thus showing 

 that the fourth quartet arose by dexiotrojiic cleavage. As may be 

 seen from the spindles in fig. 15, the tm-ning of the spindle itself is very 

 slight indeed. 



As figs. 20, 25, 26 and 30 show, these lower cells are very small com- 

 pared to the cells from which they arose. Wilson (98) has designated 

 these small cells the macromeres, and the large cells from which they 

 arose the foiu'th tiuartet. As will become evident later on, I have 



