PRELIMINARY ACCOUNT OF THE CELL LINEAGE 



OF PLANORBIS. 



SAMUEL J. HOLMES. 



THE eggs of Planorbis are deposited in capsules which 

 are usually found fastened by a viscid secretion to stones or 

 aquatic plants. In P. trivolvis, the species studied, the period 

 during which eggs are laid extends from early spring until late 

 in the fall. The eggs contain somewhat less than the usual 

 amount of yolk ; there is a clear, protoplasmic area at the 

 animal pole, while the lower half of the egg is composed prin- 

 cipally of deutoplasm. 



In the matter of nomenclature I have followed Conklin in 

 designating the different cell generations given off from the 

 macromeres by coefficients instead of exponents. For instance, 

 la would represent a cell of the first generation of ectomeres, 

 20. one of the second, and so on. 



At the first cleavage the egg is divided into equal blasto- 

 meres, and the four-cell stage is produced by the almost simul- 

 taneous division of these two blastomeres into subequal cells. 

 This division takes place in a right-handed spiral ; two of the 

 cells B and D meet in a cross furrow at the vegetative pole, 

 while the other two blastomeres, A and C, come in contact 

 above. The first cleavage furrow is oblique to the future 

 longitudinal axis of the embryo ; a plane cutting the centers 

 of B and D nearly coincides with the future sagittal plane. A 

 cleavage cavity makes its appearance in the two-cell stage, 

 reaching its maximum size immediately before the division 

 into four cells. Small cavities commonly occur between the 

 blastomeres during several subsequent stages of cleavage. 



The first quartette of ectomeres is given off in a left-handed 

 spiral, the cells \a and ic meeting in a cross furrow at the 

 apical pole. This division is followed by a dexiotropic cleav- 

 age of the macromeres, and soon afterward the first genera- 

 tion divides in a dexiotropic direction, thus giving rise to a 



