No. 2.] THE CELL LIiVEAGE OF P LA NO RBI S. IOI 



give rise to the shell gland and the median portion at least of 

 the foot. In fact the cells of the posterior quadrant of the 

 second quartette, with the quadrants a and*/ of the third, give 

 rise to the larger part of the body of the embryo. 



The cells of the fourth quartette undergo a third division in 

 a more or less equatorial direction and the small cells A, B, and 

 C divide, forming a fifth quartette. The number of entomeres 

 increases still further before gastrulation begins. An embolic 

 gastrula is formed, the blastopore becoming an elongated, slit- 

 like orifice which closes from behind forwards. The lips of 

 the blastopore close for a short time, but the definitive mouth 

 makes its appearance, at a slightly later stage, at the point of 

 closure. Whether or not an actual fusion of the lips of the 

 blastopore occurs cannot at present be stated. The regions in 

 front and behind the blastopore and at the sides are derived 

 from the second quartette, while the cells of the third quar- 



* 



tette lie at the angles. In the region around the mouth the 

 cells become large and clear, and this area is continuous pos- 

 teriorly with a narrow median band of clear cells separating 

 the halves of the fundament of the foot. 



The two mesomeres soon after the division of the primary 

 mesoblastic cell come to lie entirely in the cleavage cavity 

 at the posterior side of the egg. Their first division is very 

 unequal and seems to have been overlooked by Rabl. Each 

 mesomere gives off at its anterior end a minute cell, not larger 

 than the first polar body. These cells are clear and lie close 

 together in the cleavage cavity ; their further history has not 

 been followed. At the next division of the mesomeres begins 

 the formation of the mesoblastic bands described by Rabl. 



Fuller details of the subject of this short sketch will be 

 given in a future paper. It is a source of pleasure to acknowl- 

 edge the many favors granted me by Prof. C. O. Whitman in 

 connection with this work and the valuable suggestions I have 

 received from Dr. E. G. Conklin. 



UNIVERSITY OF CHICAGO, 

 April 19, 1897. 



