86 



John H. Gkrould, 



4. Maturation and Fertilization of the Egg. 



The dissolution of the germinal vesicle and the formation of 

 the first polar spindle (Fig-. 3—7) take place in the body cavity, 

 before the eggs are taken into the nephridia. The polarity of the 

 egg now is made evident by the appearance of translucent, finely 

 granular protoplasm at the active pole, which is destined to become 

 the centre of the apical plate of the trochophore. As in Nereis, 

 this region is sharply marked off from the yolk-containing cytoplasm 

 (Fig. 14, 15, 17, 19). It extends from the active pole into the 

 centre of the egg, where, in the earliest stage observed, the spindle 

 of the first polar globule, already in the metaphase, lays in a radial 

 position near the surface (Fig. 6). 



First Period: Entrance of the Spermatozoon. The 

 egg without further nuclear changes is thrown out into the sea 

 water. Although spermatozoa may surround the egg at once, about 

 fifteen minutes usually elapse before one has penetrated through a 

 pore of the thick yolk membrane into the cytoplasm. The penetration 

 of the spermatozoon is perhaps retarded by the presence of the layer 

 of protoplasmic processes, which project through the pore canals of 

 the zona radiata in the unfertilized egg, and which are reti-acted 

 into the cytoplasm at the time of fertilization. Upon entrance into 

 the cytoplasm, the sperm-head (Fig. 8, 13) immediately rotates to a 

 position in which its long axis is parallel to the surface of the egg. 

 At this time, a small aster containing a minute centrosome appears 

 at the base of the bullet-shaped head, while the acrosome is still 

 visible at the apex (Fig. 13). It is evident from the position of the 

 centrosome that this arises in the cytoplasm in the vicinity of the 

 middle piece. 



SecondPeriod: Giving off of First Polar Body (15— 25 

 minutes after approach of spermatozoon). During the next ten minutes 

 the undivided astrosphere, with the nucleus following it, passes to 

 the centre of the egg, leaving behind it in some instances a pathi 

 of visibly modified protoplasm (Fig. 14). The radiating fibres of the 

 sperm aster increase gradually in prominence, and a deeply-staining' 

 filament of about the same length as the diameter of the nucleus 

 connects the latter with the centrosome (Fig. 10, 15). This deeply- 

 staining rod or filament is of two or three times the thickness ofl 

 one of the astral rays. A similar structure has been noted inj 

 Thalassema by Gritfin (1899). Achromatic astral filaments extending^ 



