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AN AMERICAN TENT-BOOK OF PHYSIOLOGY. 



existing (Fig. 224, c). The two take positions beside the nucleus (Fig. 224, 

 d) and await the time when they can exert their specific function. AVe have 

 spoken of the difference of opinion regarding the origin of the original cen- 

 fcrosome of fertilization. The origin of the two centrosonies present in seg- 

 mentation has likewise been disputed. The question is of considerable the- 

 oreticaJ interest in connection with the problem of the physical basis of 



Pig 224.— Stages in the fertilization oJ the egg (continued from Fig. 223). 



minuti - after entrance of the spermatozoon. The male and the female pronuclei have met 

 in a r the centre of the egg and the fusion 1ms begun ; the former has become enlarged and its chromatin 

 has become loosely reticulated. The sperm-aster has become enormously enlarged. The single centro- 

 some has been divided into two, which lie upon either side of the Bperm-nucleus. 



d. "-till later after entrance of the spermatozoon. The two pronuclei have united to form the firsl 

 segmentation-nucleus. The sperm-aster bas become divided into two asters, which have moved to 

 opposite poles of the nucleus. The i gg is nov ready to undergo segmentation. 



inheritance. Certain observers have claimed that the centrosomes have a 

 double origin, one being derived from the male and one from the female 

 germ-cell. Upon this theory sexuality is shown by the cytoplasmic centro- 

 somes as well as by the nuclear chromosomes, and the inference is possible that 

 cytoplasm, as well as nucleus, transmits hereditary qualities. This double 

 origin of the centrosomes is not supported by trustworthy evidence. Oilier 

 observers, following Boveri, maintain that the centrosome of the sperma- 



