122 CONK LIN. [Vol. XIII. 



In summing up the history of the second quartette attention 

 should be called to the fact that in the gasteropods, which have 

 been studied with reference to the cell lineage, there is no 

 marked difference in size between the members of this quar- 

 tette, and accordingly the divisions up to the time when there 

 are eleven cells in each quadrant are almost identically the 

 same in each of the four groups. On the other hand, in the 

 annelids generally and in Unio, at least, among the mollusks, 

 the posterior member, 2d, is much larger than any of the others; 

 its divisions are bilateral and more numerous than in the cor- 

 responding cells of the other quadrants. This difference seems 

 to be due to a shortening of the development and a consequent 

 precocity in the segregation of materials destined to form the 

 principal organs of the body. There is evidence, as will be 

 shown later, that essentially the same organs develop from the 

 cell 2d in Unio and Crepidula ; the difference therefore in this 

 case is not one of material substance or destiny but rather a 

 time difference according to which the development of 2d in 

 Unio is compressed into a much smaller number of cleavages 

 than in the case of Crepidula. 



2. The Third Quartette. 



All that has been said of the difficulties of tracing the cells 

 of the second quartette is true in still greater degree of those 

 of the third. The early divisions of this quartette are much 

 slower than those of the second, and there are no distinguish- 

 ing marks by which the cells may be known. I have therefore 

 been unable to trace this quartette beyond the stage in which 

 it gives rise to six cells in each quadrant, or twenty-four cells 

 in all. 



The first division of this quartette occurs at the stage when 

 there are twenty-nine cells present. Before the division has 

 been completed in all the quadrants the first quartette has 

 divided twice and the second three times. The cleavage 

 is not simultaneous in all the quadrants, the order of divi- 

 sion being 3d, 3c, 3b, 3a (Figs. 25-28). The direction of 

 the cleavage is nearly radial, though after the cleavage has 



