44 THE MATURATION OF THE EGG OF THE MOUSE. 



SECOND POLAR CELL. 



The shape of the second polar cell is similar to that of the first, 

 though it is perhaps more often uniformly regular in shape. In order 

 to compare the size of the second polar cell with that of the first, meas- 

 urements were made of as many newly formed polar cells as possible 

 (Stage X, fig. 30). Since the condition of the polar cells during cleavage 

 stages of the egg has not been studied, changes in size are not here con- 

 sidered. For convenience the sizes of the first polar cells (exclusive of 

 the small degenerate forms) are repeated in this connection (table 6). 



Table 6. — Size of polar cells. 

 Average dimensions of first polar cell. Mi era. 



Newly formed polar cell (first spindle in 



telophase) 22.7X19.2X13.5 



From eggs but a short time in the oviduct 



(complete second spindle) 20 X15.6X11.8 



From eggs after 29 hours in the oviduct. .. 16 X13 X10.5 



Average dimensions of second polar cell. 

 Newly formed 19. 3X16. 7X 9.6 



When first produced the second polar cell, then, is smaller than the 

 first polar cell of corresponding age, but is larger than the first polar cell 

 which has been in existence for 29 hours or more. 



At the beginning, the chromatin of the second polar cell is in a 

 single mass, as in the case of the first polar cell, but it does not long 

 remain so, for it is quickly transformed into a nucleus. 



The cytoplasm in the recently cut off cell (fig. 30) has the clear 

 appearance noted in the case of the first polar cell, but later it generally 

 has the aspect of the protoplasm of the egg. The interzonal filaments 

 persist for a time and can be observed joining the nucleus of the polar 

 cell with that of the egg, the cell plate remaining as a conspicuous, 

 deeply stained body outside both egg and polar cell. 



The position of the second polar cell with regard to the first (when 

 the latter is present) is variable, for the two polar cells may lie side 

 by side or be far apart. The reason is probably to be found in the migra- 

 tion of the first polar cell, as discussed on page 63. The second polar 

 cell, like the first, occupies a slight depression in the surface of the egg. 



