No. I.] THE EMBRYOLOGY OF THE UNIONIDAE. 21 



by its general form. The embryo has the appearance of an 

 irregular mosaic. 



After the establishment of the germ-layers in separate cells, 

 and before the beginning of bilateral divisions, a fourth division 

 of the first somatoblast takes place. A small cell {x'', Fig. 39) 

 is budded off anteriorly towards the vegetative pole and against 

 the posterior end of the second somatoblast. This division 

 of X does not occur in this form in Nereis. The fourth 

 division in Nereis is equal and bilateral, whereas in Unio the 

 fifth is the first bilateral division. Other divisions take place 

 at about this time on or in the region of the vegetative pole, 

 which give it a most characteristic appearance. These are the 

 divisions of x^, d^ and F. ;tr' buds off a small cell towards the 

 antero-lateral border of the second somatoblast (-f •' and x^ 

 Figs. 39 and 40). A division of exactly the same general 

 form takes place in Nereis (No. 64, Figs. 52 to 54). d^ divides 

 somewhat later (<^3i and ^3% Figs. 40, 42, and 45 ; d'^-^ helps 

 in the overgrowth of M). This division of d^ is interesting 

 from the fact that the other micromeres of the third generation 

 do not divide till much later, d'^ has apparently inherited the 

 tendency of its parent macromere D to rapid division. Y buds 

 off y\ between d^, A and a^ (PI. IV, Figs. 39 and 40), and a 

 little later _;/^ on the other side (PI. V, Fig. 59). 



While dealing with these divisions it will be just as well to 

 include the observations which I have made on the further 

 divisions of the first and second groups of micromeres. If the 

 order of division were the order of description it would be 

 necessary to postpone this for some time later, but in that 

 case, I fear that the reader would be as tangled up in the 

 description as I was at one time in the apparently confused 

 and indeterminate order of the facts. I have followed the 

 first group of ectomeres to a stage when sixteen cells of this 

 group are formed. It would have required much time and 

 trouble to have followed them farther: and, inasmuch as no 

 larval apical organs are formed, I desisted. I have not, there- 

 fore, found the cross on which Conklin lays so much stress in 

 Crepidula ; I doubt very much its existence in any stage in 

 Unio, for I should certainly have seen it, were it formed. 



