THE FERTILIZATION OF THE OVUM. 17 



believed that they were derived " from the division of the egg 

 nucleus after it had given rise to the second polar body" (p. 60). 

 As described by Van Beneden, the position of the attraction 

 spheres at their earliest appearance, is shown in Fig. i A. 

 They appear together in close proximity to the female pro- 

 nucleus and at a considerable distance from the male pro- 

 nucleus. Van Beneden therefore supposed that the male 

 pronucleus had nothing to do with their formation. In later 

 stages, Fig. i, B and C, the two spheres begin to separate, 

 the spindle fibres appearing between their central corpuscles 

 and at the same time the two pronuclei approach each other 

 and the first cleavage spindle is formed, in large part at least, 

 from the two attraction spheres. 



One year later, 1888, Boveri 1 described these bodies in 

 the same egg, that of Ascaris ; the central corpuscle of Van 

 Beneden he called the centrosome, and the dark granular sub- 

 stance surrounding this, which probably corresponds to the 

 cortical zone of Van Beneden, he named the arcJioplasm. The 

 relation of these parts to each other, and to the pronuclei, is 

 shown in Fig. i, D, E and F, which are taken from Boveri. 

 When first seen in the ovum, the male pronucleus lies in the 

 midst of a mass of granular material, the archoplasm. Later 

 it moves out of this, and in the place where it first lay a single 

 highly refractive body, the centrosome, appears. The centro- 

 some is at first single but later it divides, as shown in E, and 

 then around each of the centrosomes the archoplasm aggregates 

 to form two granular spheres. Meanwhile the two pronuclei 

 enlarge greatly and approach each other while the spindle 

 fibres are formed from the two spheres of archoplasm. With 

 regard to the origin of the constituents of the archoplasmic 

 system, Boveri believed that the centrosome was derived from 

 the spermatozoon, since it first appears in the archoplasm at 

 the very place previously occupied by the male pronucleus, 

 while the archoplasm itself, he supposed, came entirely or at 

 least in large part from the egg cell. 2 



1 Boveri, Zellen-Studien, Heft 2, Die Befruchtung und Theilung des Eies von 

 Ascaris megalocephala, 1888. 



2 Loc. cit., p. 167. 



