No. 2.] EARLY DEVELOPMENT OF PLANORBIS. 409 



The next divisions in this quartette occur in the cells 3^'', 

 3<3:'"'', 3^'"', 3^'■^ on the posterior side of the Q.gg, and in 3<$''*, 

 3(5""^ 3c-^*', 3^^"^ on the anterior side. These divisions are radial 

 and similar in character, each cell budding off a small cell toward 

 the vegetal pole. There thus arise four pairs of small cells, the 

 two anterior pairs lying in the angles between the entomeres 

 on either side of the median plane, the two posterior pairs lying 

 above the large cells, 3«^ and 3^". The upper pair of cells in 

 each of the four quartettes next divide in the same direction 

 as before, forming a vertical series of four pairs of cells in the 

 anterior quadrants, and a similar series of three pairs of cells 

 above the large cells, 3^^ and ^cT, in the posterior quadrants. 

 The third quartette now contains thirty cells, eight in each 

 anterior quadrant, and seven in each posterior one. 



The two pairs of cells, 3<^^''"', 3^^'^S 3^^''', and 3^""^"', become 

 pushed in towards the cleavage cavity and become partly cov- 

 ered by the surrounding cells. They divide in a nearly hori- 

 zontal direction, and their daughter-cells become pushed into 

 the cleavage cavity still further. They form an irregular row 

 of four small cells lying above the pairs of small cells in the 

 angles between the entomeres. The period of their division is 

 quite variable. In one case (PI. XIX, Fig. 27) a division has 

 evidently occurred in 3^^'' before the upper cells have divided, 

 but this does not usually occur. In PI. XIX, Fig. 33, 3^^'^'' has 

 divided, while the cell lying beside it is entire, as are also the 

 corresponding cells in the b quadrant. The lower products of 

 the division of the uppermost cells in the anterior quadrants, 

 3<^''''^ 3<^'■^■^ 3^'■'■^ 3<^'■'■^ divide in a nearly radial direction. 

 PI. XIX, Fig. 33, shows 3^*''' dividing, while the cleavage in 

 3^''^''' is completed. The number of cells in each anterior quad- 

 rant is now twelve ; the small cells in the angles between the 

 entomeres have remained undivided since their origin ; above 

 this pair are the four cells, which are partly sunk into the cleav- 

 age cavity, arranged in a transverse row, and above these again 

 are three pairs of cells in a vertical series. The number of cells 

 in each posterior quadrant is still seven, and the whole number 

 of cells in the third quartette is thirty-eight. The entire tg'g 

 contains at this period about 150 cells. The four cells in each 



