No. I.] THE EMBRYOLOGY OF THE UNIONIDAE. 35 



described the mesoblast as formed by the third cleavage of the 

 posterior macromere. E. B. Wilson has already pointed out 

 his probable error. In all of the other cases cited the meso- 

 blast is formed by the fourth cleavage of the posterior or left 

 posterior macromere. Heymons (No. 47, p. 271) has already 

 emphasized this fact. 



The accompanying diagrams will serve to emphasize the facts 

 already dwelt on and to bring out others. The first diagram 

 illustrates one point of fundamental importance, viz. : That the 

 posterior macromere contains all the elements of the trunk in 

 Clepsine Nereis,^ Amphitrite, and Unio. The first somatoblast 

 (neuro-nephroblast, Whitman) forms the ventral plate and all 

 ectomermal elements of the trunk ; while the second somato- 

 blast forms the mesodermal elements of the trunk. (In no 

 other mollusc (Diagram 2) has the ectoderm of the trunk been 

 traced to the posterior ectomere of the second generation. But 

 from an inspection of Heymons' and Conklin's figures I have 

 but little doubt that the shell-gland at least is derived from this 

 blastomere ; the cytogeny of the foot is not as yet known in 

 these forms.) It is probable, too, that in all these cases the 

 members of the first generation of ectomeres form the region 

 in front of the prototroch. This has been shown beyond doubt 

 for Nereis (two sp.) Umbrella, Crepidula Unio, and Amphitrite, 

 and made more than merely probable for other forms. In 

 all cases, too, the second and third generations of ectomeres 

 probably take up homologous positions in the body. The 

 macromeres after the separation of the ectomeres and mesoderm 

 (by the same number of cleavages) become entomeres. 



of ectomeres is invariable (Wilson, No. 64 ; Heymons, No. 47) in Annelida and 

 Mollusca ; all micromeres after the third generation, with the exception of the 

 posterior micromere of the fourth generation (mesoblast), being entodermic. From 

 Brobetzky's description of Nassa one would conclude that more than three genera- 

 tions of micromeres are formed ; but until it is shown that all are ectomeres, we 

 cannot accept this as evidence against the views upheld here. Similarly it is not 

 certain as yet just how many generations of ectomeres are formed in Clepsine. 

 However, it has not yet been shown for any annelidan or molluscan ovum of 

 the oblique type of cleavage that more or less than three generations of ectomeres 

 is formed. 



^ In Nereis a dorsolateral strip of trunk epiblast is derived from A and C, 

 according to Wilson. 



