602 HAROLD HEATH, 



appearance to those in Ischnochiton, yet the direction of the spindles 

 is slightly difierent. But even though this be the case a general 

 resemblance may be traced beyond this point and in the position of 

 the arms the two forms are closely similar as far as they have been 

 traced. 



The posterior arm is rapidly modified from its radial condition 

 as the divisions of the basal and tip cells are teloblastic, ultimately 

 giving rise to a long slender arm one cell wide extending below the 

 level of the velum. The division of the tip cell therefore is different 

 from those in the remaining quadrants and from those in Ischnochiton 

 and the subsequent cleavages have no resemblance in the two forms. 

 It is interesting to note that after many divisions in the posterior 

 arm of Ischnochiton^ which in general resemble those of other quadrants, 

 the cells arrange themselves in rows extending from the rosette series 

 down to the prototroch (Fig. 45). They have much the appearance 

 of having been formed by teloblastic divisions but primarily their 

 arrangement is not due to such a cause. 



This apparently affords another example of the effects of a 

 bilateral symmetry upon the earlier stages. In forms as widely 

 separated as TJnio and Nereis or Amphitrite, for example, there is an 

 excessive development of the posterior second and fourth quartette 

 cells (first and second somatoblasts) with correlated modifications, and 

 the above resemblance between the posterior arms in Crepidula and 

 Ischnochiton belongs to the same category. In such cases however it 

 by no means follows that the cleavages which produce such characters 

 as the ventral plate and teloblastic arrangement of cells are identical. 

 At another time I shall advance some reasons for believing that while 

 precocious segregation tends to produce an early appearance of a 

 bilateral condition it tends to destroy close cell homologies. And in 

 this case of the posterior arms we find that there is a tendency ex- 

 pressed to depart from the radial condition and to assume a telo- 

 blastic arrangement although the cleavages producing these cells are 

 not similar. 



3. Rosette series — "Annelid Cross". 



The first cells of this group arise in the 24 cell stage by a dexio- 

 tropic division. Each of these products divides by a leiotropic 

 cleavage into two cells (Fig. 17) ; the central group of 4 constitute 

 the apical series and the 4 peripheral the rosette series (Wilson). 



The next division occurs in the two posterior cells of the rosette 



