MATURATION OF THE EGG, AND PROCESS OF FERTILISATION. 33 



nucleolus, and not separated from the protoplasm by any fixed 

 membrane. Similar distinctions in the condition of the germinative 

 vesicle and the egg-nucleus recur throughout the animal kingdom. 



The formation of polar cells, and the accompanying metamorphosis 

 of the germinative vesicle into such an extraordinarily reduced egg- 

 nucleus, is a phenomenon of very wide, probably, indeed, of general 

 occurrence. Polar cells have been observed throughout the Coelen- 

 terates, Echinoderms, Worms, and Molluscs. In the ripening of the 

 eggs of Arthropods, according to the earlier observations, they 

 appeared never to be present; but recently they have been found in 



Fig. 15. 



Fig. 14,-Mature egg of an 1 chinoderm. It encloses in the yolk the very small homogeneous- 



egg-nucleus (fc). 

 Fig. 15. Immature egg from the ovary of an Echinoderm. 



numerous species by a number of observers, especially by BLOCHMANN 

 and WEISMANN. Among Vertebrates polar cells are always en- 

 countered in Cyclostomes and Mammals, whereas in Fishes and 

 Amphibia they have been identified only in some cases, and in Reptiles 

 and Birds not at all as yet. They arise either some time before or 

 else during fertilisation. 



In the case of Mammals (Rabbit and Mouse) the process has been 

 very carefully investigated by VAN BENEDEN, and recently by TAFANI. 

 Sovera. weeks before the rupture of the GRAAFIAN follicle the ger- 

 minative vesicle ascends to the surface of the egg ; some days before 

 that epoch it there disappears, and at the place where it disappeared 

 there are formed the egg-nucleus and, under the zona pellucida, one 

 or two (TAFANI) polar cells. The egg after it has escaped from the 

 ovary always exhibits egg-nucleus and polar cells. 



Also in the case of Fishes, Amphibia, Reptiles, and Birds, whose 



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