328 PROCEEDINGS OF THE ACADEMY OF [April, 



This stain gives a reddish tint which differentiates the nuclei with great 

 distinctness. Iron haematoxylin proved entirely unsatisfactory for 

 sections of both early and late stages, for even in the old veligers almost 

 all the cells are found to contain small yolk spherules which take up 

 the haematoxyhn so strongly and hold it so tenaciously that nuclei and 

 cell walls are indistinguishable. Eggs which have just been stained 

 and mounted are not favorable objects for study, but they should, 

 if possible, stand for some time, the longer the better, until they gradu- 

 ally become more transparent by the penetration of balsam. Indeed, 

 the most favorable slides are a few put up at the time the material 

 was collected. By the addition of a little cedar oil to the balsam, or 

 by moistening the edges of the cover with xylol at the time of using, 

 it is always possible to roll the eggs by moving the cover — a very 

 necessary'- process in cell-lineage work. Most of the observation and 

 drawing was done with the aid of a Leitz objective 7, ocular 4, a 

 Zeiss camera being used, with the paper at table level and plates re- 

 duced as indicated. A -^ Leitz immersion was also used for obser- 

 vation when necsseary. 



Nomenclature. 



As a matter of convenience and for the sake of uniformity, I have 

 followed the system used by Conklin (1897) ^vith but slight variation. 



A cleavage is obHque to the right when the upper daughter cell lies 

 to the right of an imaginary observer whose body corresponds in posi- 

 tion to the primary egg axis, his head being at the animal pole and 

 facing the cell considered; vice versa, a division oblique to the left is 

 one in which the upper cell lies to the observer's left. In the first 

 instance the cleavage is dexiotropic, in the second loeotropic (Lillie, 1895). 



The term "quartet" is used to designate a generation of cells or 

 their derivatives given off from the four cells meeting in the center of 

 the vegetative pole, regardless of their fate. The different quartets 

 are designated by coefficients placed before the letter indicating in 

 wliich of the four quadrants the cells lie, while the cell generations are 

 marked by exponents which follow the letter. The upper cell resulting 

 from a cleavage is, in all cases, indicated by the smaller exponent; 

 thus, 2b" indicates the upper cell in B quadrant of the second quartet 

 arising from the division of 2b\ while 2b^^ is the lower. "When the 

 spindle lies in a horizontal direction or, in other words, when the cleav- 

 age plane is meridional, the cell which lies to the right is given the 

 smaller exponent, to the left the larger. The capital letters A, B, C, 

 and D are reserved for the four cells which meet at the center of the 



