CHAP. I INTEODUCTION 9 



are exhibited also by the oocyte of the first order, as we term the egg 

 when it has reached its full size. Its nucleus passes through the 

 stages of leptonema, zygonema, pachynema, strepsinema, etc. The 

 nucleolus disappears, the nuclear membrane dissolves, and the nuclear 

 sap mingles with the cytoplasm. The consequence of this is, that 

 since the chromosomes form a small and inconspicuous mass, the egg, 

 viewed under the low power of the microscope, appears to have lost 

 its nucleus, and this is a ready way to distinguish unripe from ripe 

 eggs, or in other words, oogonia from oocytes of the first order. 



The great difference between oocytes and spermatocytes of the 

 first order is that in the case of the latter when the cell divides, it 

 gives rise to two daughter cells of the same size which form 

 spermatocytes of the second order, but when the oocyte divides, it 

 forms two daughters of unequal size, the larger forming the oocyte 

 of the second order whilst the smaller forms a rudimentary cell 

 incapable of development, termed the first polar body. At the 

 second maturation division the same phenomenon repeats itself, the 

 oocyte of the second order divides into two unequal daughters — the 

 larger is the ripe ovum, whilst the second is again a rudimentary 

 cell which never develops and which is termed the second polar 

 body. The first polar body often divides into two daughters of 

 equal size which are, like the second polar body, to be regarded as 

 sisters of the egg, or better, as rudimentary eggs (Figs. 3, B, and 4, B). 

 The same sacrifice of quantity to quality therefore which is seen in 

 the absorption of many oogonia by their more fortunate sisters, repeats 

 itself in the maturation divisions. 



The ripe egg can now receive the spermatozoon. As soon as the 

 head of the first spermatozoon has penetrated the egg an alteration of 

 the surface of the latter usually takes place, which cuts off the tail of the 

 spermatozoon and prevents any other spermatozoa from entering; 

 the middle piece of the successful spermatozoon, however, foUows the 

 head into the egg. This alteration in the surface of the egg has 

 been diagnosed by Loeb as a kind of cytolysis : for it can be observed 

 that a number of fine globules issue from the surface of the egg,, and 

 that their outer surfaces coalesce to form the inner egg-shell or vitelline 

 membrane. 



Meanwhile the spermatozoon head within the ovum swells up, it 

 assumes a vesicular form, and nuclear membrane, ceU. sap and 

 chromosomes can be again demonstrated in it. The middle'piece 

 takes on the character of a centrosome and around it the achromatic 

 rays appear, forming what is termed an "aster." The male pro- 

 nucleus, as the spermatozoon head is now termed, moves towards the 

 residual nucleus of the egg, termed the female pronucleus, which in 

 its turn advances towards the male. Male and female pronuclei 

 then meet and fuse and form a single nucleus, the zygote nucleus, 

 and fertilization is complete. 



After a resting period of an hour or two the zygote nucleus 

 Ijegins to prepare for karyokinesis, and the spindle is so formed that 



