118 



embryos go on simultaneously in the two individuals. Each 

 nucleus forms an embryo — only one nucleus to each embryo — and 

 the latter develops two or three chambers by growth before it 

 escapes from the conjugal enclosure. 



It is therefore clear that, whilst an agamic reproduction is the 

 commonest method of increase among the Foraminifera, a conjugal 

 union of individuals is necessary at certain times and under fit 

 conditions as a means of preventing the deterioration of the 

 species. It has also been established, so far as the species placed 

 under observation are concerned, that the act of copulation is 

 exclusively of the nature of plastogamy. 



Quite recently Mr. J. J. Lister has propounded a very ingeni- 

 ous theory to explain the alteration of plan of growth which takes 

 place in the microspheric forms of Biloculina and Triloculina* 

 Among the Miliolidae the principal types exhibit a biloculine, 

 triloculine, or quinqueloculine test, according as two, three, or 

 five chambers are exposed externally. It has been observed that 

 in the case of the (? sexually produced) microspheric forms of 

 Triloculina the early chambers of the shell are arranged on a 

 quinqueloculine plan, changing in the later stages to the trilocu- 

 line arrangement; and in the case of Biloculina, the early 

 chambers are quinqueloculine, then triloculine, and finally 

 biloculine. No such transmutations of form occur in the 

 (asexual) megalospheric forms, but these are respectively either 

 triloculine or biloculine throughout their growth. The questions 

 Mr. Lister has attempted to solve are — First, why this remark- 

 able change should take place in the growth of the genera referred 

 to ; and, second, why such a change should be characteristic of 

 the microspheric and not the megalospheric form. The assump- 

 tion is, that the sexually-produced microspheric form goes out of 

 its way to repeat the arrangement characteristic of allied forms 

 before it attains the arrangement proper to its own genus. Mr. 

 Lister says — " Is not this a particular instance of a phenomenon 

 widely met with in higher forms of animals, in which the indi- 

 viduals produced by budding attain the adult structure by a 

 direct development, while those produced from the egg often 

 develop by an indirect course, going out of their way to repeat 

 lost features characteristic of the archaic forms of their race 1 . . 

 In the case of higher animals the larval stages are lost, the body 

 of the larva being fashioned into that of the adult, but in this 

 group of the Protozoa, the Miliolidse, the peculiar structure of the 

 young is permanently recorded, being built in and retained in the 

 centre of the chambers subsequently added, "f If Mr. Lister's 



* " A Possible Explanation of the Quinqueloculine arrangement of the 

 Chambers in the Young of the Microspheric Forms of Triloculina and 

 Biloculina," Proc. Cambridge Phil. Soc, vol. IX., p. 236 (1897). 



t Op. cit. 



