No. I.] THE EMBRYOLOGY OF THE UNIONIDAE. 9 



system proves the difficulty to be real. Yet, if the best results 

 are to be obtained from the comparison of the cell-lineages of 

 different animals, some fairly uniform system of nomenclature 

 must be adopted by the different workers in the field. For 

 the sake of uniformity I have adopted the system followed by 

 E. B. Wilson in his "Cell-Lineage of Nereis." ^ 



The first four cells are designated by the capital letters A, 

 B, C, and D ; the generations of ectomeres by the small letters 

 a, b, c, d; the first index number indicates the generation to 

 which the ectomeres belong: thus, «' or ^'-^ or <:'" all belong 

 to the first generation, c% d^\ a"-^ belong to the second genera- 

 tion, and so forth. A, B, C, and D and a\ b\ c\ and d^ are 

 retained throughout for the four vegetative and four apical pole 

 cells, respectively. When a cell divides, the products receive 

 the designation of the parent cell with the addition of a further 



index number: thus, a^<^ ^^ ; the larger index 2 is used for 



the cell lying nearer the vegetative pole. Exceptions to this 

 rule are made only in the cases of important cells, which 

 receive special designations. Thus the first somatoblast is d^; 

 this designation is replaced by X, and the small cells formed 

 from it by x\ x^, etc. The mesoblast is indicated by M for the 

 teloblast and m for the smaller cells ; in the case of the larval 

 mesoblast the designation a^-^ is replaced by Y. For the rest, 

 the system will develop as the account proceeds. 



Another matter which demands a preliminary explanation is 

 the orientation given the figures of segmentation. I have 

 oriented them all with the part, usually placed below, above. 

 The reason for this is that it obviates the confusing changes 

 of orientation which would otherwise be necessary. The chief 

 part of the large (posterior) cell gives rise to the shell gland, 

 which is dorsal ; in keeping it above throughout I have had in 

 mind the final orientation of the embryo. This is (for the 

 same reason) the orientation which has been used by other 

 writers on the Unionidae. At the close of the work there is 

 inserted a section on the change of axes. 



1 The system of nomenclature lately proposed by Mr. Kofoid (No. 49 a) is 

 too rigidly symmetrical to be applicable to a cleavage so irregular as that of Unio. 



