136 THE FORAMINIFERA 



contrast may be found in the difference in the mode of origin 

 of the two forms. 



The life-history of the Cladoceran Leptodora hyalina, appears to 

 offer a similar contrast. Throughout the summer months broods 

 of young are produced, which develop parthenogenetically and 

 are hatched in the form of the parent. The resting winter 

 egg, on the other hand, which develops as the result of fertilisa- 

 tion, emerges as a Nauplius larva the form in which the members 

 of such diverse families take their origin, and which, there is good 

 reason to believe, repeats in several of its features the characters 

 of the primitive Crustacea. 



In the case of Leptodora we see that after, and apparently as the 

 result of, fertilisation the organism " casts back " in its develop- 

 ment, repeating primitive features which are abbreviated or absent 

 in the development of the form arising without a sexual process. 

 Now although the megalosphere of the Foraminifera, the product 

 of the multiple fission of the parent, may not be strictly comparable 

 with the unfertilised egg of Leptodora, it has, at least, this in 

 common with it, that it arises asexually, while it is probable that 

 the microspheric form arises from the conjugation of gametes, a 

 process comparable to the fertilisation of the Metazoa. In the 

 paper referred to it was suggested that the accentuation of the 

 multiform character of the microspheric form of the Foraminifera, 

 as compared with the megalospheric, is likewise dependent on the 

 process of fertilisation. 



It still appears to me possible that the explanation may 

 be found in the direction indicated, but that this is not the 

 complete solution is shown by consideration of the Initial Poly- 

 morphism displayed by the megalospheric forms of several 

 species. In Idalina and Orbiculina we have seen that the extent 

 to which the phases of growth which occur in the development of 

 the microspheric form are repeated by the megalospheric form 

 varies in different individuals, and that it is correlated with the 

 size of the megalosphere individuals with small megalospheres 

 repeating these phases more completely than those with large 

 megalospheres. What the cause of this correlation may be 

 appears entirely obscure, but it is evident that if among the 

 megalospheric forms, arising asexually, the completeness of the 

 repetition of the earlier phases depends on the size of the central 

 chamber, we are not at liberty to refer the completeness of 

 their repetition in the microspheric form wholly to its sexual 

 origin. 



In his sketch of a natural classification of the Foraminifera (36 and 37) 

 Rhumbler takes altogether different views of the phenomena we have 

 been considering, and the classification proposed as the result has been 

 adopted by Lang in the new edition of his Lehrbuch. 



