422 Dr. A. Borgert on the Reproduction 



LXIII. — On the Reproduction of the TripyJean Radiolaria 

 {Phceodaria). By Dr. A. BORGERT *. 



During my sojourn tliis year at the Zoological Station at 

 Naples there was afforded to me in the months from January 

 to May the opportunity, vainly sought for in former years, of 

 studying more closely the conditions of reproduction in the 

 Pha^odaria. The special subject of my investigations was 

 Aulacantha scoJi/mantha. In the case of this form I succeeded 

 in observing direct as well as mitotic nuclear division and in 

 following in detail the processes that take place therein. 



In direct nuclear division we observe no gradual constric- 

 tion, but rather a cleavage of the nucleus into two halves of 

 approximately equal size. The cleft, which originally is 

 quite narrow, grows broader in subsequent stages, and the 

 two halves of the nucleus separate one from the other and 

 become rounded off. These changes in the interior of the 

 central capsule are accompanied by others which are external. 



Thus we note the appearance of an annular shallow groove 

 with sharply defined edges on the surface of the central 

 capsule. This groove lies in the same plane as the cleft 

 passing through the nucleus, i. e. vertically to the frontal 

 plane f, and divides the main aperture. Instead of the single 

 main aperture originally present two such apertures are 

 formed. Between the edges of the groove the capsular 

 membrane is very delicate. It therefore easily happens that 

 in isolating a central capsule in process of division the two 

 halves come apart in the groove. This is also the spot at 

 which in binary fission the severance of the daughter capsules 

 takes place. Each of these individuals has to supply a 

 secondary aperture by new formation, since in the process of 

 fission it receives only one. Certain figures published by 

 B. Hertwig J and Haeckel § undoubtedly refer to the direct 

 mode of division. Similarly it is evident that Karawaiew 1|, 

 during his investigations upon Aulacantha^ frequently ob- 



* Translated by E. E. Austen from the ' Zoologischer Anzeiger,' 

 Bd. xix. DO. 507 (July 6, 1896), pp. 307-311. 



t I follow the example of Haeckel in designating by the term " frontal " 

 plane that in which are situated the three apertures of the central capsule. 



X R. Ilertwig, 'Der Organismus der Radiolarien,' 1879, Taf. ix. tig. 2, 

 Taf. X. fig. 2. 



§ p]. Haeckel, " Report on the Radiolaria collected by H.M.S. ' Chal- 

 lenger ' during the years 1873-1876," ' Challenger ' Reports, Zoology, 

 vol. xviii. pi. ci. tig. 2. 



II W. Karawaiew, " Beobachtungen iiber die Structur und Vermehrung 

 von Aulacantha scoh/mantha, Haeckel," Zool. Anz, Bd. xviii. (1895) 

 pp. 286-289 and 29^^301. 



