THE CELL LINEAGE OF PODARKE OBSCURA. 



PRELIMINAR Y COMMUNICA TION. 

 A. L. TREADWELL. 



THE segmentation of Podarke is of especial interest as a 

 representative of the "equal type" of cleavage among anne- 

 lids. By the first three cleavages the ovum is divided into 

 eight cells of equal size, four above and four below. The 

 next division establishes inequalities in size among the cells, 

 but until the 56-cell stage is reached the quadrants are exactly 

 alike. At the 56-cell stage one quadrant becomes different 

 from the other three, and soon after bilateral divisions begin. 



The transition from the 2 to the 4-cell stage is accompanied 

 by a rotation to the left, and the familiar cross furrows appear. 

 The furrow at the top is at right angles to that at the bottom 

 and much shorter. The furrow at the bottom has the same 

 direction that it has in Nereis, 1 Amphitrite, 2 Arenicola, 3 and 

 Crepidula, 4 and retains this direction until it is possible to 

 distinguish the quadrants by means of other landmarks. Later, 

 owing probably to variations in position of the cells forming it, 

 the direction of this furrow may vary. The first two planes 

 of cleavage have the same relation to the median axis of the 

 embryo that they have in Amphitrite and Arenicola. 



The second group of micromeres 5 are given off in a left- 

 handed spiral. These cells arise at the same time and are 

 equal in size, there being no large d 2 . Simultaneously with 

 the origin of these, the cells a 1 ' 1 , b l '\ c l '\ </ I-I are formed at 

 the upper pole. 



In the transition from sixteen to thirty-two cells all the divi- 



1 E. B. Wilson, " Cell Lineage of Nereis," Journ. of Morph., vol. vi. 



- A. D. Mead, " Early Development of Marine Annelids," Journ. of Morph., 

 vol. xiii. 



3 C. M. Child, Zodl. Bull., vol. i, no. 2. 



* E. G. Conklin, " Embryology of Crepidula," Journ. of Morpli., vol. xiii. 



5 The term " micromeres " is used here simply for convenience, the division of 

 the " macromeres " being approximately equal during the first four divisions. 



