POLAR GLOBULES IN WHITE MICE. 



313 



formed , it is developed in the ovum as it lies in the tube. When two globules are 

 formed, the nuclear spindles of the first and second differ somewhat in appear- 

 ance ; when only one is formed, it resembles that of the second globule. Hence 

 we are led to surmise that there may have been in these cases really a first polar 

 globule formed, but that it appeared so much earlier in the history of the cell that 

 it escaped observation. 



The First Polar Globule. — The first polar globule is formed, as stated, while 

 the ovum is still in the unruptured Graafian follicle of the ovary. The nucleus 

 moves toward one side of the ovum and is there transformed into a mitotic 

 spindle, the axis of which is more or less nearly at right angles to the radius of the 

 ovum (Fig. 178). The spindle itself is large, pointed at the ends, with curving 

 achromatic threads. The chromosomes, which are probably twelve in number, 

 gather themselves into an equatorial plate. They are elongated, pointed at the 

 ends, with irregular sides, and are very large. No trace 

 of the centrosome has been observed at the end of the 

 spindle and there are no astral rays extending from the 

 ends of the spindle into the protoplasm. The chrom- 

 osomes are somewhat V-shaped. They divide by a 

 transverse separation at the apex of the V. Chrom- 

 osome halves migrate toward the end of the spindle. 

 These stages occur probably about twenty-four hours 

 before the rupture of the follicle. The actual extrusion 

 of the first globule has not yet been described. It 

 takes place before ovulation, for the first polar globule 

 is always found while the ovum is still in the ovary. 

 In the mouse it is remarkable, as is also the second 



polar globule, for its large size. It is usually oval or spherical in form, and may 

 measure say 16 or 17 // in length by 9 ft in diameter. It has a distinct cell 

 membrane, a protoplasm which resembles that of the ovum, and may even con- 

 tain granules of yolk. Soon after its separation from the ovum its nucleus be- 

 comes well developed and membranate. Except, therefore, that the number of 

 chromosomes which enter into its formation is half the normal number, we 

 might say that it differs little from an ordinary cell. 



The Second Polar Globule. — After the formation of the first polar globule we 

 do not find the nucleus of the ovum entering into a condition of repose, but it at 

 once transforms itself, as in other animals, into the second polar spindle. The 

 form and changes through which this passes are similar to those of the spindle 

 in the case in which only a single globule is found, to the description of which we 

 now pass. 



Fig. 178.— Ovum of White 

 Mouse, with the First 

 Polar Spindle in Tan- 

 gential Position. X 5°° 

 diams. — (Af/er J. Sobotta.) 



