MATURATION AND IMPREGNATION OF THE OVUM. Ill 



pearance of the germinal vesicle they differ considerably as to 

 the manner of this occurrence. Some are of opinion that the 

 vesicle simply vanishes, its contents being absorbed in the ovum ; 

 others that it is ejected from the ovum and appears as the polar 

 cell or body, or Ricktungskorper of the Germans — a small body 

 which is often found situated in the space between the ovum and 

 its membrane, and derives its name from retaining a constant 

 position in relation to the ovum, and thus serving as a guide in 

 determining the similar parts of the embryo through the different 

 stages. The researches of Oellacher (15)^ in this direction deserve 

 special mention, as having in a sense formed the foundation of the 

 modern views upon this subject. By a series of careful observations 

 upon the egg of the trout and subsequently of the bird, he demon- 

 strated that the germinal vesicle of the ovum, while still in the 

 ovary, underwent partial degeneration and eventually became 

 ejected. His observations were made to a great extent by means 

 of sections, and the general accuracy of his results is fairly cer- 

 tain, but the nature of the eggs he worked on, as well as other 

 causes, prevented his obtaining so deep an insight into the phe- 

 nomena accompanying the ejection of the germinal vesicle as has 

 since been possible, Loven, Flemming (6), and others have been 

 led by their investigations to adopt views similar in the main to 

 Oellacher's. As a rule, however, it is held by believers in the 

 disappearance of the germinal vesicle that it becomes simply ab- 

 sorbed, and many very accurate accounts, so far as they go, have 

 been given of the gradual atrophy of the germinal vesicle. The 

 description of Kleinenberg (l4) for Hydra, and Gotte for Bom- 

 binator, may perhaps be selected as especially complete in this 

 respect ; in both instances the germinal vesicle commences to 

 atrophy at a relatively early period. 



Coming to the more modern period the researches of five 

 workers, viz. Biitschli, E., van Beneden, Fol, Hertwig, and 

 Strasburger have especially thrown light upon this difficult sub- 

 ject. It is now hardly open to doubt that while part of the 

 germinal vesicle is concerned in the formation of the polar cell 

 or cells, when such are present, and is therefore ejected from the 

 ovum, part also remains in the ovum and forms a nuclear body 

 which will be spoken of as \)[it female pronucleus, the fate of which 

 is recorded in the second part of this paper. The researches of 

 Biitschli and van Beneden have been especially instrumental in 

 demonstrating the relation between the polar bodies and the ger- 

 minal vesicle, and those of Hertwig and Fol, in showing that part 

 of the germinal vesicle remained in the ovum. It must not, 

 however, be supposed that the results of these authors are fully sub- 



' The numbers appended to authors' uames refer to the list of publica- 

 tions at the end of the paper. 



