The Development of Ischnochiton. Gil 



state to what extent they enter into the formation of the shell. The 

 portion of the first somatoblast below the growth zone consists of a 

 row of cells which extend to the blastopore, and for a considerable 

 length of time (during the first stages of invagination) they occupy 

 the bottom of a groove whose sides are formed by the larger third 

 quartette cells. Later as this latter quartette becomes composed of 

 cells of smaller size the groove disappears and this part of the 

 somatoblast becomes ill-defined. 



What part the somatoblast plays in the development of the ventral 

 surface it is impossible to state, but no doubt it forms a much smaller 

 portion than the third quartette. It is probable that in such forms 

 as Amphitrite, where the entire trunk forms from 2d, the ventral 

 nerve cord arises from this cell. It may do so in Ischnochiton but 

 since this part of the nervous system has probably been modified it 

 does not necessarily follow that it arises from the same cell in 

 difierent groups of animals (cf. page 649). However, there is no doubt 

 but that the somatoblast in Ischnochiton forms part of the shell, a 

 small portion of the posterior part of the body, including mantle and 

 mantle furrow, and a portion of the foot. 



Comparison s. 



In Ischnochiton it is readily seen that the first somatoblast arises 

 from a cell which in origin, size and position is similar to the other 

 second quartette cells, and that in all probability its relatively ex- 

 cessive development has been secondarily acquired owing to the shifting 

 of the mouth and formation of a ventral surface. Consequently it is 

 reasonable to suppose that in the original radial ancestor the second 

 quartette developed in the same manner in each quadrant. 



Among Molluscs, Crepidula shows more strikingly than Ischno- 

 chiton the radial symmetry of the second quartette. As far as the 

 cell lineage has been followed (up to a time when eleven cells in each 

 quadrant have been formed) the changes have been similar in all 

 quadrants. Beyond this point differences between the posterior group 

 and the three remaining arise, but the manner in which this is brought 

 about is not known. 



In Annelids whose cleavage belongs to the "equal type" (Lepido- 

 notus, Podarke) there is at first no distinction in size among the 

 second quartette cells, and Tread well has shown for Podarke that 

 the first divergence in development occurs in the posterior group 

 upon the formation of four cells in each quadrant. At this time the 



