34 PROTOZOA 



cases which prevail among the Higher Animals and Plants, the 

 larger cell is motionless, and the smaller is active, ciliate, flagellate, 

 or amoeboid: the coupled -cell or zygote is here termed the 

 " oosperm." ^ It encysts immediately in most Protista except 

 Infusoria, Acystosporidae, Haemosporidae, and Trypanosomatidae. 

 As the size of the two gametes is so disproportionate in most 

 cases that the oosphere may be millions of times bigger than the 

 sperm, and the latter at its entrance into the oosphere entirely 

 escape unaided vision, the term " egg " is applied in lax speech, 

 both (1) to the female cell, and (2) to the oosperm, the latter 

 being distinguished as the " fertilised egg," a survival from the 

 time when the union of two cells, as the essence of the process, 

 was not understood. 



AVe know tliat in many cases, and have a right to infer that in all, 

 chemiotaxy plays an important part in attracting the pairing-cells to one 

 another. In Mammals and Sauropsida there seems also to he a rheotactic 

 action of the cilia lining the oviducts, which work downwards, and so induce 

 the sperms to swim upwards to meet the ovum, a condition of things that 

 was most puzzling until the nature of rheotaxy was understood. A remark- 

 able fact is that equal gametes rarely appear to be attracted by members ot 

 the same brood, though they are attracted by those of any other brood of the 

 same species.^ It may well be that each brood has its own characteristic 

 secretion, or " smell," as it were, slightly different from that of other broods, 

 just as every dog has his, so easily recognisable by other dogs ; and that the 

 cells only react to different " smells " to their own. Such a secretion from 

 the surface of the female cell would lessen its surface tension, and thereby 

 render easier the penetration of the sperm into its substance. 



As a rule, one at least of the pair-cells is fresh from division, and it would 

 thus appear that the union of the nuclei is facilitated when one at least of 

 them is a " young " one. Of the final mechanism of the union of the nuclei, we 

 know nothing : they may unite in any of the earlier phases of mitosis, or 

 even in the "resting state." A fibrillation of the cytoplasm during the 

 process, radiating around a centrosome or two centrosomes indicates a strained 

 condition.^ 



■^ For details see Hartog, "Some Problems of Reproduction," Quart. Journ. Micr. 

 Sci. x.xxiii. p. 1, xlvii. p. 583; and Ami. Biol. vol. iv. (1895) 1897 ; E. B. Wilson, 

 Yves Delage, and Henneguy (references on p. 3, note) ; and for a singularly clear 

 and full treatment of the processes in Protozoa, Arnold Lang, Lehrb. d. Vergl. 

 Jnat. 2nd ed. Lief. 2, "Protozoa," 1900. 



- This phenomenon, which we have termed "exogamy," is common in Proto- 

 phyta ; it has been clearly demonstrated by Schaudinn in Foi-aminif'era and the 

 Lobose Rhizopod Triclwsjihaerium (p. 53 f. Fig. 9), and by Pringsheim in the 

 Volvocine Pandorina (p. 128 f. Fig. 45). It is quite independent of the differentia- 

 tion of binary sex. 



' Other modes of syngamy, such as karyogamy and plastogamy, we shall discuss 

 below, pp. 69, 148 ; see also p. 30. 



