J. F. McCLENDON. 



have been made on Ascaris eggs by Auerbach ('74) ; on am- 

 phibian eggs by Pfluger ('84), Roux ('85, '93), Born ('93, '94), 

 O. Hertwig ('93) ; on echinoderm eggs by Driesch ('92, '93, '95), 

 Morgan ('93), Ziegler ('94) ; on ctenophore eggs by Ziegler 

 ('94); and on Nereis eggs by Wilson ('95). These experi- 

 ments show that if the egg is pressed 

 more on certain sides than on others, as 

 when it is pressed between two plates of 

 glass and forced to assume a flattened 

 shape, that the direction of the cleavage 

 spindles (and consequently cleavage fur- 

 rows) will be affected. Hertwig formu- 

 lated the law that the spindle lies in the 

 longest axis of the protoplasmic mass of 

 the cell. This rule probably applies in 

 the majority of cases, but there may be 

 some exceptions, and there are evidently 

 other and unknown factors which enter 

 into the polar differentiation of the cell. 

 Bigelow found in Lepas ('02) that the 

 polarity of the egg was not affected by 

 the oval form of the rigid chorion. The 

 principal axis of the egg coincided with 

 the long axis when the chorion was se- 

 creted and during the prophase of the 

 first cleavage spindle the egg rotated 



through a right angle so that the first cleavage spindle was made 

 to coincide with the long axis of the egg determined by the form 

 of the chorion, and the principal or primary axis was perpen- 

 dicular to it. 



I found a similar condition in Endactylina nigra Wilson. If the 

 egg string of this copepod be placed in sea water under the micro- 

 scope during the first cleavage stage, the majority of the eggs will 

 have their spindle axes, or in case the division is complete, common 

 cell axis, nearly in the same plane. Often, however, some of these 

 axes are considerably inclined to this plane as is shown in Fig. 4, A. 

 If the egg string be pressed between slide and cover glass the above 

 axes will rotate sufficiently to bring them all in a plane midway 



FIG. 4. Egg strings of 

 Endaclylina nigra Wilson. 

 The protoplasm is stippled 

 and the yolk white, the dis- 

 tinctness between the two 

 being accentuated. A, Liv- 

 ing egg string during first 

 cleavage. B. The same com- 

 pressed between the slide 

 and cover glass. 



