MATURATION OF THE EGG, AND PROCESS OF FERTILISATION. 43 



of its nuclear substance, which was described as a small, deeply 

 stainable spherule, there arises a vesicular nucleus (fig. 25 sk), which 

 acquires the same size and condition as the egg-nucleus. 



Fig. 22. 



Fig. 23. 



Fig. 22. An egg of Ascaris megalocephala just fertilised, after VAX BENEDEN. 



sk, Spermatic body, with nucleus, which has entered the egg ; /, fat-like substance of the 

 spermatic body ; kb, germinative vesicle. 



Fig. 23. A stage of a fertilised egg of Ascaris megalocephala. somewhat older than that of 



fig. 22, after VAN BENEDEN. 

 .;/,-, Spermatic body, which has penetrated deeper into the cortex of the yolk ; sp, polar spindle 



which has arisen from the germinative vesicle ; ch, chromosomes of the spindle. 



After the rapid and continuous accomplishment of these processes, 

 the egg of the Worm usually enters on a longer or shorter period of 



Fig. 25. 



Fig. 24. A still older stage of development, following that of fig. 23, of the egg of Ascaris 



megalocephala, after BOVERI. 

 up, Polar spindle, which has ascended to the surface of the yolk ; ch, 2 x 4 chromosomes; 



ak, spermatic nucleus, which has migrated into the middle of the egg. 



Fig. 25. Egg of Ascaris megalocephala in preparation for the process of cleavage, after 



. E. VAN BENEDEN. 

 2>2, Two polar cells which have arisen from the polar spindle (sp~) of fig. 24 by a repetition 



of the process of budding ; ei, egg-nucleus ; sk, spermatic nucleus already preparing to 



divide ; ch, nuclear loops or chromosomes. 



rest. It now presents (compare fig. 25, which represents a stage 

 already further developed) at its surface within the vitelline mem- 

 brane two polar cells (pz), and in its interior two large vesicular 

 nuclei, the spermatic nucleus (sk) and the egg-nucleus (ei), the 



