12 



MATURATION OF OVUM 



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der Stricht in the bat), 1 and it has now long been known that the process by 



which each polar body is formed is in reality a cell-division, in which the 



nucleus is equally, but the pro- 

 toplasm very unequally, divided. 

 They are thus minute cells, 

 often called the pole-cells, 

 and are regarded as abortive 

 ova. In some cases the first 

 polar body undergoes division ; 

 when this occurs, the typical 

 group of four becomes formed. 

 Such groups are of frequent 

 occurrence in the produc- 

 tion of the gametes, not 

 only in the animal, but also 

 in the vegetable kingdom. Only 

 one of the four is, however, 

 functional. 



The oocyte after the first 

 division is called the oocyte of 

 the second order ; and when this 

 again divides, and the second 

 polar body is thrown off, the 

 egg is said to be mature. The 

 essential difference is that the 

 large vesicular nucleus of the 

 oocyte of the first order has been 

 converted into a smaller nu- 

 cleus, which retires to the centre 



of the egg, there to await the advent of the sperm-nucleus in fertilisation. It is to 



the egg at this final stage that, strictly speaking, the term ovum should be applied 

 During the maturation of the ovum important nuclear phenomena present 



themselves, which are essentially similar to those which take place in the 



sperm-cells, as was first proved 



by Oscar Hertwig in his classical 



researches on Ascaris megalocephala, 



published in 1890. The full signifi- 



cance of these nuclear changes can- 



not, however, be made clear until 



the process of fertilisation is under- 



stood. The treatment of the his- 



tory of the nucleus during matu- 



ration will therefore be deferred 



until that process has been studied. 



" 



FIG. 19. HUMAN OVUM AT END OF GBOWTH-PERIOD. 

 (Van der Stricht.) 



Showing distribution of mitochondria in deutoplasmic 

 zone, with vacuoles, fat-drops, yolk-granules, &c. 



FIG. 20. THE POLAE BODIES AND EGG-NUCLEUS ; 



ECHINUS ESCULENTUS. (T. H. Bryce.) 



Only a portion of the ovum is represented. 



x 1200 diameters. 



In the process of spermato- 

 genesis the centrosome, it will be 

 recollected, persists and is related to the flagellum, but in the case of the egg 

 it in many instances seems wholly to disappear during maturation. This has been 

 made the basis of certain theories of fertilisation which will be alluded to later. 



In the above account the case has been described in which maturation is 

 completed before the egg leaves the ovary. In many animals the spermatozoon 



Anat. Anzeiger, Erganzungsheft, xxvii. 1905. 



