POLAR GLOBULES IN WHITE MICE. 



161 



proximal part of the tube, forming, as it were, a special sac, with a distended 

 epithelial lining. At the time of coitus ovulation has generally taken place; the 

 ovum, still surrounded by the cells of the corona radiata, is found in the fluid of 

 the distended proximal section of the tube. It is probable that the ova are carried 

 from the periovarial space not only by the currents created by the cilia of the 

 fimbriate opening, but also by a sort of pumping action of the tube itself. For 

 at the beginning of the period of heat we find that the periovarial space contains 

 much fluid, but later, when the ova are in the tube, this space is empty and the 

 tube contains fluid. The ovum of the mouse measures only 80^ or less in 

 diameter, and is therefore the smallest known mammalian ovum. (The ovum of 

 the cat measures 200^, of the rabbit 161^.) It is surrounded by a very thin 

 zona pellucida (i6-36//), and contains only a few yolk grains, a portion of which 

 may be blackened by osmic acid. These ova offer the further special peculiarity 

 that the first polar globule, which is always formed in the ovary, is lost in 80-90 

 per cent of the ova, probably by extrusion through the zona pellucida, so that 

 even after the formation of the second globule, they still often have Only a single 

 globule within the zona. The second globule is produced only after the ovum has 

 been transferred to the uterine tube, and then only after a spermatozoon has 

 entered. The process for formation of the first and second globules is not the 

 same, although there is a general similarity. 



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. 114). The spindle 

 itself is large, pointed at the ends, with curving achromatic 

 threads. The chromosomes are probably twelve in number, 

 but they vary in size and shape, and even in number, which 

 has been explained as the result of precocious division of some 

 of them. They gather themselves into an equatorial plate. FlG - 

 They are elongated, pointed at the ends, with irregular sides, 

 and are very large. Minute centrioles have been observed at 

 the end of the spindle, but there are no astral rays extending 

 from the ends of the spindle into the protoplasm. The chro- 

 mosomes become somewhat V-shaped. They divide by a trans- 

 verse separation at the apex of the V. Chromosome halves migrate toward the end 

 of the spindle. The stages occur probably about twenty-four hours before the 

 rupture of the follicle. The spindle now assumes a radial position, and one of its 

 poles lies close to the surface of the ovum, which has meanwhile diminished in size 

 so that there is a considerable space between the yolk and the zona pellucida. 

 Division occurs and the first polar globule is formed, and lies in the perivitelline 

 space. In the mouse it is remarkable, as is also the second, but smaller, polar 



114. OVUM OF 

 WHITE- MOUSE, WITH 

 THE FIRST POLAR 

 SPINDLE IN TANGEN- 

 TIAL POSITION. X 500 

 diams. (After J. 

 Sobotta.) 



