Ill 



FORAMINIFERA 



69 



only exogamously} The fusion of their nuclei takes place after 

 some delay: ultimately the zygote nucleus divides into two, a 

 shell is formed, and we have the microsphere, which is thus 

 pluri- nucleate ab initio. As we have seen, the nuclei of the 

 microsphere are ultimately replaced by chromidia, and the whole 

 plasmic body divides into pseudopodiospores, which grow into the 

 megalospheric form. 



In the Perforate genera, Patellina and Discorbina, plastogamy 

 precedes brood formation, the cytoplasms of the 2-5 pairing 

 individuals contracting a close union ; and then the nuclei 

 proceed to break up without fusion, while the cytoplasm 



Fig. 17. Shell of Ulobigerina bulloides, from tow-net, showing investment of spines. 

 (From Wyville Thomson.) 



aggregates around the young nuclei to form amoebulae, which 

 acquire a shell and separate. In both cases it is the forms 

 with a single nucleus, corresponding to megalospheric forms that 

 so pair, and the brood-formation is, mutatis mutandis, the same 

 as in these forms. Similar individuals may reproduce in the 

 same way, in both genera, without this plastogamic pairing, which 

 is therefore, though probably advantageous, not essential. If 

 pseudopodiospores form their shells while near one another, they 

 may coalesce to form monsters, as often happens in Orbitolites. 2 



The direct economic uses of the Foraminifera are perhaps 

 greater than those of any other group of Protozoa. The Chalk is 



1 Which probably accounts for the earlier failure of Lister and of Schaudinn 

 himself to note their conjugation. 



2 Rhumbler, " Die Doppelschalen v. Orbitolites u. and. Foraminiferen," in Arch. 

 Protist. i. 1902, p. 193. 



