MATURATION AND IMPREGNATION OF THE OVUM. 523 



of the embryo. (2) According to the other set it remains in 

 the ovum and by successive divisions forms the parent nucleus 

 of all the nuclei in the body of the embryo. Though the second 

 of these views has been supported by several very distinguished 

 names the first view was without doubt the one most generally 

 entertained, and Haeckel (though from his own observations 

 he was originally a supporter of the second view) has even 

 enunciated the theory that there exists an anuclear stage, 

 after the disappearance of the germinal vesicle, which he regards 

 as an embryonic repetition of the monad condition of the 

 Protozoa. 



While the supporters of the first view agree as to the dis- 

 appearance 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 Ricktungskb'rper 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 (I5) 1 in this direction 

 deserve special mention, as having in a sense formed the founda- 

 tion 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 demonstrated 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 certain, but the nature of the eggs he worked 

 on, as well as other causes, prevented his obtaining so deep 

 an insight into the phenomena 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 absorbed, and many very accurate 



1 The numbers appended to authors' names refer to the list of publications at the 

 end of the paper. 



342 



