Fig. I. — Diagram of the cleavage in Clepsine, Nereis, Amphitrite, and Umo. The part in 

 square brackets is peculiar to Unio. In Clepsine the third generation of ectomeres has not been 

 found and the mesoderm is formed frofn the posterior ma. 3. — ec. i., ec. 2., ec. 3., first, second, and 

 third generations of ectomeres. ma. 1. to ma. 4., Macromeres of different stages, en., Entomere. 

 Larv. mes.. Larval mesoblast. som. i., First somatoblast. som. 2., Second somatoblast. 



Fig. a. — Diagnm of the clearage in Crepidula, Umbrella, Neritina, Planorbis, Limax, and Phjrta. 



