64 THE MATURATION OF THE EGG OF THE MOUSE. 



DIVISION OF SECOND SPINDLE AND ABSTRICTION OF SECOND POLAR CELL. 



The only illustrations showing the division of the second spindle 

 in the maturation of the mouse egg are those of Sobotta (1895, 1907). 

 The criticisms which we have made in connection with the division of 

 the first spindle and the formation of the first polar cell (p. 59) are appli- 

 cable to the corresponding processes in the second oocyte. 



4. POLAR CELLS. 



There is agreement among the investigators of the mouse egg that 

 not all fertilized eggs have both polar cells. According to Tafani and 

 Gerlach the first polar cell is always formed, but the second in a large 

 proportion (respectively four-fifths and three-fourths) of the eggs is sup- 

 pressed. Tafani does not state how the suppression is effected. Gerlach 

 thinks that in the event of late semination the second spindle divides 

 so quickly as to inhibit the formation of the polar cell and that the chro- 

 mosomes which would have been contained in the second polar cell 

 remain in the cytoplasm of the egg and degenerate. Although he avers 

 that he has seen such degenerating chromatin, it should be borne in 

 mind that it is possible he mistook for chromosomes cytoplasmic bodies 

 which sometimes stain deeply like chromatin. Sobotta, on the other 

 hand, believes that in most cases the first polar cell is never formed. 

 In 1895 he stated that even the first spindle did not come into existence. 

 Now (1907) he believes that the spindle is formed in all eggs, but that in 

 4 out of 5 eggs it is immediately metamorphosed into the second spindle, 

 half of the chromatin disintegrating in the egg. As he has not seen 

 either the metamorphosis or the degeneration of the chromatin he has 

 no direct evidence for his belief. Kirkham states, but on evidence that 

 in part at least is unsound, that all eggs produce the first polar cell. 

 His explanation (19076, p. 80) of the absence of one polar cell is appar- 

 ently suggested by a single case in the bat, in which, according to van 

 der Stricht, both polar cells lay outside of the zona pellucida. It is 

 supported by one observation (Kirkham, 19076, p. 81), according to 

 which the polar body of a living mouse egg (which he stained and dehy- 

 drated under the microscope) was forced through the zona pellucida by 

 the contraction of the latter under the influence of changing osmotic con- 

 ditions. 



While the case in the bat is suggestive of a possible explanation for 

 the loss of the first polar cell in the mouse, it can scarcely be admitted 

 as evidence of the occurrence of such conditions in the mouse. As for his 

 observation on the living egg, Kirkham does not say with what strength 

 of solutions he stained and dehydrated the egg under the microscope. 

 Although he may have seen the polar cell forced through the zona under 

 direct action of reagents, the same thing need not necessarily occur under 

 natural conditions, since eggs in the oviduct, and still more those in the 

 ovary, are protected from the full vigor of osmotic action by the sur- 



