No. 2.] EARLY DEVELOPMENT OF PLANORBIS. 40I 



elongated in the opposite direction and takes part in the form- 

 ation of the head vesicle. The elongation of this cell goes 

 hand in hand with the growth of the posterior trochoblasts, 

 whose increase in size would naturally subject it to a lateral 

 pressure. The peculiar history of this cell is evidently corre- 

 lated with the large size which the head vesicle attains in this 

 form. It affords another case of precocious specialization of 

 function such as occurs in the trochoblasts. In fact, the fate 

 of this cell and the posterior trochoblasts is identical, as they 

 all go to form the same organ. The head vesicle in Planorbis 

 develops early, and reaches a much larger size than it attains 

 in Crepidula. The posterior tip cell becomes differentiated at 

 an earlier period and stops dividing. In Crepidula it divides 

 twice, perhaps many times, and the products of these divisions 

 remain for a long time apparently little modified. Their pre- 

 cise fate is uncertain ; probably some of them, at least, enlarge 

 and enter into the formation of the head vesicle, as in Planorbis. 



The other tip cells divide twice in Crepidula, forming a row 

 of four small cells lying across the tips of the arms of the cross. 

 All of these cells, accordingly, go into the upper row of cells, 

 forming the prototroch. Except in the anterior arm of the 

 cross, the tip cells in Planorbis do not divide at all. The 

 lateral tip cells, like the posterior one, enlarge, become trans- 

 parent, and go to form a part of the head vesicle. There is 

 probably no other group of cells in these two forms which 

 present such marked differences of behavior as the tip cells of 

 the cross. In Crepidula they are at first very small and of 

 unequal size; they grow very little and divide several times. 

 In Planorbis they are at first quite large and of equal size ; 

 they grow quite rapidly, and, with the exception of the anterior 

 one, never divide at all. In Crepidula those of the anterior and 

 lateral arms go into the prototroch ; in Planorbis only the ante- 

 rior one goes into the formation of this organ, and this cell, as 

 far as could be determined, undergoes only one division. 



The next cleavage, after the stage to which the history of the 

 cross has been traced, occurs in the four cells lying in the angles 

 between the arms. These divisions are bilateral ; in the ante- 

 rior pair of cells the left cell divides dexiotropically, the right 



