1 84 CONK LIN. [Vol. XIII. 



next bilateral cleavage occurs at the 42-cell stage, and it also 

 consists of a slight change in the direction of division of the 

 single cell 2d2. The first bilateral division in the mesoblast 

 occurs at the 44-cell stage, and all subsequent divisions in this 

 layer are bilateral. From this time on bilateral cleavages 

 increase in number, but up to the stage with 1 1 1 cells perfect 

 spiral cleavages are present, and in the very latest stages to 

 which any group of cells could be traced spiral cleavages were 

 found in some of the cells (usually three) of each group. (See 

 table of Directions of Cleavage.) 



That the reason assigned by Wilson for the " bilateral asym- 

 metry " of the early stages is not applicable here is shown by 

 the fact that in many of the gasteropods the left posterior mac- 

 romere is not appreciably larger than the right and in some 

 {e.g.. Umbrella) it is smaller, and also by the fact that the 

 mesoblast (4d) is only one member of a quartette which is sep- 

 arated in a left spiral from the macromeres, each of the other 

 members being quite as large as, or even larger than, the cell 

 4d. The following conclusions may be drawn concerning the 

 origin of bilateral cleavages among the gasteropods : 



(i) Bilateral cleavages first appear on the posterior side of 

 the egg. 



(2) They are generally due to a reversal of the direction of 

 cleavage of one out of four cells, this reversal being most 

 frequent in quadrant D. 



(3) Certain time differences appear between the divisions on 

 the anterior and posterior sides of the Q.g^, the divisions on the 

 posterior side being much slower in the first quartette, but 

 ultimately much more rapid in the second and third. 



(4) Another factor in the establishment of bilaterality, and 

 the one which gives meaning to the three preceding ones, 

 is the teloblastic growth of all the layers at the posterior end 

 of the embryo and the formation in this region of the larger 

 part of the adult body. 



(5) The primitive radial symmetry is preserved in the ante- 

 rior quadrants long after it has disappeared in the posterior 

 ones, e.g., the arms of the cross, the origin of larval mesoblast 

 in quadrants A, B, and C, etc. 



