No. 2.] EARLY DEVELOPMENT OF PLANORBIS. 407 



remains entire. In both Umbrella and Crepidula the divisions 

 of the second quartette, as far as they have been traced, are, 

 with the exception of the cleavage of the tip cell in the latter 

 form (and, possibly, also in the former), perfectly similar in 

 every quadrant. This may be due to the fact that the forward 

 rotation of the apical pole occurs later in these forms. The fact 

 that when this rotation is delayed the similar character of the 

 division of the quadrants of the second quartette is maintained 

 for a longer period, lends additional support to the view that in 

 Planorbis the delayed cleavage in the anterior quadrant is caus- 

 ally connected with this rotation. The early beginning of this 

 rotation in Planorbis, caused as it is by the more rapid growth 

 of the posterior trochoblasts and the posterior arm of the cross, 

 may be viewed as a result of the early formation of the head 

 vesicle. This structure develops early in Planorbis, and reaches 

 a larger size than in Crepidula or Umbrella. The different be- 

 havior of the cells of the anterior quadrant of the second quar- 

 tette in these different gasteropods may thus be considered a 

 sort of indirect effect of the different degrees of development 

 which the head vesicle attains in these forms. 



The Mesoblastic Bands. 



The mesoblastic bands in Planorbis have been fully described 

 by Rabl, who was the first to derive the mesoderm in the gas- 

 teropods from a single cell. A division occurs, however, after 

 the first cleavage of the primary mesomere, which it seems that 

 Rabl overlooked. After the two mesomeres have come to lie 

 entirely in the cleavage cavity, each buds off at the anterior end 

 a minute clear cell, which is often quite difficult to observe. 

 The spindles are inclined slightly towards each other at their 

 anterior ends, and the small cells that arise lie almost in con- 

 tact with each other. The next cleavage of the mesomeres is 

 horizontal and equal, and at right angles to the preceding divi- 

 sion. There thus result an inner and an outer pair of large 

 mesodermic cells, which are figured by Rabl in Figs. I'ja and 

 17^. The inner pair of cells are the mesoblastic teloblasts. 

 The next cleavage of the teloblasts is in the same direction as 



