148 JOHN PRIESTLEY. 



In their accounts of the final dissolution, however, they 

 diiFer. In Hertwig's view the whole structure, after having 

 become peripheral, disappears, leaving behind it of its con- 

 tents merely the germinal spot, which remains as the nucleus 

 of the ovum {Eikern). Van Beneden thinks that the walls of 

 the peripherally-seated vesicle fuse with the nucleolus or ger- 

 minal spot, and the body thus formed, together with the re- 

 mains of the reticular threads and pseudo-nucleoli, which 

 constitute another body, are rejected as so-called Richtungs- 

 hldschen or corps directeurs. 



In this connection mention must be made of the state- 

 ments of Balfour^ in a monograph on the developmental 

 history of Elasmobranch Fishes, which he is now publishing. 

 He believes'that the vesicle atrophies as the ovum ripens, and 

 that its contents become indistinguishable from the surround- 

 ing protoplasm. The membrane, in the cases where it is 

 thick and resistant, may escape complete absorption and be 

 extruded bodily, as in the instances of osseous and elasmo- 

 branch fishes, birds, &c.; but in the majority of ova it is quite 

 absorbed, although it may appear again in the guise of the 

 Richtungs-Korper. 



The fate of the germinal vesicle has always excited great 

 interest among embryologists, and three different views exist 

 concerning it. According to the first, which, as we have seen, 

 is shared by Auerbach and Strasburger, the vesicle dis- 

 appears entirely. Purkinje^ and von Baer^ both describe 

 it as moving towards the periphery of the ovum while still 

 in the oviduct, and as finally becoming ruptured and dis- 

 appearing ; and in this description they have been followed 

 by many observers in more recent times. 



According to the second view, the germinal vesicle con- 

 tinues as such, and undergoes division prior to that of the 

 whole cell. This, which is supported in special cases by 

 statements of Joh. Miiller, Leydig, Gegenbauer, and Fol,* 

 was maintained by van Beneden^ for the whole animal king- 



■• In the ' Journal of Anatomy and Physiology,' January, 1876. 

 2 Purkinje : ' Symbolse ad ovi avium historiam ante incubationem.' 

 ' C. E. V. Baer : ' Untersueh. ii. die Entwieklungsges. der Fische,' 1835, 

 C. E. V. Baer : ' Untersueh. ii. die Entwicklungsges. der Thiere,' Bd. fi. 



* Joh. Miiller : " Ueber die Erzeungng von Schnecken u. Holothurien," 

 ' Archiv f. Anat. ii. Phy.,' 1852. Leydig: "Ueber den Bau u. die Sys- 

 tematische Stellung der Raderthiere," 'Zeitsch. f. Wiss. Zool.,' Bd. vi. 

 Gegenbauer : " Zur Lehre vom Generat.ionswechsel u. der Fortpflanzung bei 

 Medusen u. Polypen," ' Untersueh. iiber Pteropoden u. Heteropoden.' 

 Fol : " Die Erste Eutwicklung des Geryonideneies," Jena. ' Zeitsch. f. 

 Med. u. Naturwiss.,' Bd. vii. 



* Van Beneden : ' Recherches sur la Composition et la Signification de 

 I'ceuf,' Bruxelles, 1870. 



