566 SUMMARY OF CURRENT RESEARCHES RELATING TO 



prothyalosome, a differentiated, and slightly elevated, portion of the 

 nuclear mass. 



Within the uterus of the fertilized female the zoosperms occur 

 in four forms, marking four stages of development, though all are 

 capable of fertilization. Except in the first, or simply amcBboid, 

 stage, each zooaperm consists of a granular, nucleated, cephalic hemi- 

 sphere, and of a caudal process containing a refringent body and fibrils 

 of a contractile nature. A definite membrane surrounds the " tail " 

 of the zoosperm, ending with a free border at the neck, and not 

 investing the naked protoplasm of the cephalic hemisjjhere. 



The second chapter deals with the penetration of the zoosperm 

 into the ovum, i. e. with " the copulation of the sexual products." On 

 this subject Van Beneden has observed that, in Nematodes, as a 

 general rule, only one zoosperm penetrates an ovum. The erroneous 

 view that many zoosperms entered to fertilize a single ovum, is due to 

 the presence of certain refringent bodies in the vitellus. (In Mam- 

 mals, where many zoosperms commonly penetrate an ovum, one only 

 effects fertilization, the others being assimilated as food.) The zoo- 

 sperm always enters at the micropyle, round which the membrane of 

 the ovum rises up to form a " perivitelline space." On entering, the 

 zoosperm applies itself to the " plug of impregnation " by its cephalic 

 hemisphere, — its axis being thus applied in continuation of the 

 embryonic axis of the ovum, and the homologous regions of the two 

 elements being thus brought into contact. 



Aided by its own amoeboid movements, the zoosperm is now borne 

 into the ovum centripetally by the protoplasmic process to which it 

 is applied. The membrane of the zoosperm enters into intimate 

 relations with the egg-membrane, finally fusing with it to form a 

 continuous ovo-spermatic membrane. 



In his third chapter, Van Beneden deals with the " modifications 

 which take place in the ovum from the time of copulation of the 

 sexual products to the time when the unification of the mature ovum 

 and zoosperm commences." 



In Ascaris megalocephala a Upsiliform figure represents the first 

 " directive spindle " of Biitschli (Fol's first " amphiaster de rebut "). 

 This characteristic figure consists mainly of achromatic fibrils, with 

 two chromatic disks, lying in a clear body (representing the prothya- 

 losome) at the junction of the three limbs of the Upsilon. The 

 chromatic elements are derived from the germinal corpuscle, the 

 achromatic fibrils from the germinal vesicle and its membrane. 



As the zoosperm reaches the centre of the ovum, the vertical 

 limb of the Upsilon becomes connected with it by filaments (probably 

 muscular in function), and the figure becomes T-shaped, the trans- 

 verse limb taking on the appearance of a spindle. Meantime the 

 vitellus loses all traces of its radiate structure, becoming granular 

 throughout. 



" The first polar body is now formed at the expense of the reduced 

 prothyalosome and of the chromatic elements it contains. Each of the 

 two chromatic disks furnishes to the polar body the half of its sub- 

 stance, and the prothyalosome divides tangentially. The elimination 



