FERTILISATION 15 



through the whole cell-body, and meanwhile the conical sperm-head assumes 

 a spheroidal form, and is converted into the sperm-nucleus (fig. 24, a). Several 

 observers have described the existence of a minute granule in the centre of 

 the radiation or sperm-aster, and it has been identified as the centrosome of 

 the spermatozoon. As previously mentioned, it has been supposed that the centro- 

 some disappears in the ovum during maturation. According to Boveri's theory 

 of fertilisation, the sperm-centrosome supplies a new divisional centre, and plays 

 the leading role in initiating cleavage of the egg. Wilson's ' observations, however, 

 on eggs which undergo parthenogenetic development as the result of treatment by 

 chemical substances, make it doubtful whether we can interpret the phenomena 

 in this way. His experiments prove that centrosomes may arise de novo in 

 the egg-protoplasm, and therefore it is possible that the spermatozoon produces 

 an effect on the egg-protoplasm such as to produce a centrosome, or physiological 

 centre of activity, made manifest by the radiations of the aster. 



As the result of this protoplasmic activity the sperm-nucleus now changes 

 its position and moves toward a point not quite in the centre of the egg, while 



dti 



**$^%y^^ HL, 



&*' 



FIG. 24. THREE STAGES IN THE CONJUGATION OF THE SPERM WITH THE GERM-NUCLEUS IN 

 ECHINUS ; DRAWN FROM SECTIONS, x 1200 D. (T. H. Bryce. 



at the same time the germ-nucleus is also drawn to the same point. As the nuclei 

 approach one another the aster comes into contact with the germ-nucleus, and 

 the clear area at its centre spreads over the side of that body. The nuclei then 

 conjugate (fig. 24, 6, c), the aster becomes double, and the radiations die away 

 during a pause in which the compound nucleus, or, as it is now called, the segmenta- 

 tion-nucleus, grows in size. The two asters then become related to the poles of the 

 nucleus, the radiations reappear, and the first nuclear division of the egg is 

 inaugurated. From the figures it will be clear that the nuclei conjugate while 

 in the vesicular phase, with the chromatin in the form of a network. 



The process in the mouse 2 (fig. 25) is essentially the same, but there are certain 

 variations. The spermatozoon meets the egg in the second third of the oviduct, 

 before the second pole-cell has been formed. Entrance is probably effected by the 

 piercing of the zona, and not through one of the pores in the membrane. The 

 tail seems to be cast off, and only the head and middle piece enter the egg ; but 

 in the bat the whole spermatozoon is figured by Van der Stricht as entering (fig. 26). 

 A rotation of the head occurs, and it then becomes converted into a small vesicular 

 nucleus, which is at first distinguished from the germ-nucleus by its smaller size; 



1 Archiv f. Entwickelungsmechariik, xii. 1901. 



- See Sobotta, Arch. f. mikr. Anat. xlv. 1N95. Cf. also Van der Stricht, Anat. Anzeiger, Ergiinz- 

 ungsheft, 1902, and Rubaschkin, Anat. Hefte, xix. 1905. 



