1907.] NATURAL SCIENCES OF PHILADELPHIA. 519 



In stiulying the cell lincaf^e the chief difhculty experienced was in 

 not being able to rotate the eggs under the cover glass. The eggs are 

 not orientated in any definite direction within the capsules, and it was 

 necessary to pick out for study those eggs which were favorably 

 oriented. Besides it is particularly difficult to determine the lineage 

 of certain cells if one is able to view them from one side only. The 

 fact that the eggs could not be rotated accounts for some of the draw- 

 ings being from a somewhat oblique view. 



The results obtained from studying the whole mounts w'ere checked 

 as far as possible by the use of serial sections. It was found necessary 

 to bleach the Flemming material with peroxide of hydrogen before 

 sectioning. A number of stains were used for the sections, but Dela- 

 field's haematoxlyn, either in toto or on the slide, proved most useful. 

 A combination of thionin and acid fuchsin also gave good results. 

 There is too much yolk in these eggs to use Haidenhain's iron-aluni- 

 hsematoxlyn to advantage. 



Nomenclature. 



The system of nomenclature followed in the cell lineage of this paper 

 is that used by Chabry (87), Wilson (92) and Conklin (97), wdth slight 

 modifications. This system is the same as has been used by Child 

 (1900), Treadwell (1901), Casteel (1904), Nelson (1904), and many 

 others. For the sake of convenience the chief points are repeated here. 

 Each of the four quadrants of the egg is denoted by one of the first 

 four letters of the alphabet. The left quadrant is A, the anterior B, 

 the right C, and the posterior D. The four macromeres form the basal 

 quartet ; the first group of four micromeres to be separated from these 

 is the first quartet, and so on. A micromere is denoted by a lower case 

 letter, while the capital letters are reserved for the corresponding 

 macromeres. The number of the quartet to which a micromere 

 belongs is indicated by a coefficient, while the cell generation is shown 

 by the exponent. Of the two cells of any division of a micromere 

 (except 4d), the one lying nearer the animal pole is regarded as the 

 stem cell and receives the smaller exponent. Thus 2a\ a cell of the 

 second quartet in the A quadrant, will divide into 2a*-^ and 2a^-^. 2a' ' 

 lies nearer the animal pole than 2a^-^. 2a^-^ will divide into 2a^-^-^ and 

 2a' ■^•^. In the case of the divisions of the mesentoblast, 4rf, the lower 

 cell is regarded as the stem cell and receives the smaller exponent 

 (Conklin, 97). 



A division is to the right, dexiotropic, if the upper cell lies to the 

 right of the lower when viewed by an imaginar}'' observer situatofl at 



