SYNGAMY AND SEX IN THE PROTOZOA 



127 



majority of Protozoa, as in all known cases amongst Met/azoa and 

 plants, syngamy takes the form of <karyogamy. The nuclei of 

 the gametes are termed pronuclei, and the nucleus that results 

 from fusion of the pronuclei in the zygote is termed a synkaryon. 



In many Protozoa (e.g. Coccidium, Fig. 69) the fusion of the two pronuclei 

 is effected by means of a peculiar mechanism termed a ' fertilization- 

 spindle." When the two pronuclei are in contact, the female pronucleus 

 first takes an elongated, fusiform shape, having its chromatin-grains spread 

 over an achromatinic framework. The chromatin of the male pronucleus 

 is then spread over the same structure. This mechanism has nothing to do 

 with nuclear division, but merely effects a complete intermingling of the 

 chromatin of the pronuclei, after which the synkaryon assumes its normal 

 appearance and rounded form. In Infusoria the two pronucloi fuse in the 

 condition of the karyokinetic spindle in many cases. 



FIG. 69. Fertilization of Coccidium schubergi. A, Pene- 

 tration of the macrogamete by one of five micro- 

 gametes ; the female pronucleus has an elongated 

 form ; B, the favoured microgamete has passed into 

 the interior of the macrogamete, which has secreted a 

 membrane (ob'cyst) at the surface of tho body, ex- 

 cluding the other microgamotes ; C, the female pro- 

 nucleus has assumed an elongated, spindle-like fo'nn, 

 while the male pronucleus lies at one pole of the 

 spiodle in tho form of a little mass of granules ; the 

 excluded microgametes are degenerating ; D, the granules of the male pronucleus 

 have spread themselves over the spindle-figure formed by the female pronucleus ; 

 E, the fertilization-spindle seen in D has rounded itself off to form the synkaryon. 

 and fertilization is complete. <? , Microgametes ; ? , macrogamete ; n $ , male 

 pronuclous ; w$, female pronucleus; /.., fertilization-spindle; c, oocyst ; n, 

 synkaryon. After Schaudinn (99), magnified 2,250. 



True s3Tigamy, as defined above, must be distinguished carefully from 

 certain other phenomena which are likely to be confused with it ; it must 

 not be assumed that every fusion of cells, or even of nuclei, is necessarily a 

 case of syngamy. In some Protozoa tho mother-cells of tho gametes, the 

 gametocytes, enter into a more or less close association prior to the formation 

 of gametes, 'which are produced in due course and then perform the act of 

 syngamy in the normal manner. An example of such association is seen 

 in gregarines (p. 330), where association between adult gametocytes is the 

 rule. Sometimes the two gametocytes associate in the earliest stages of 

 their growth, as in Diplocystis (Fig. 70, A], and their bodies may then fuse 

 completely into one ; but their nuclei remain distinct, as in Cystobia 

 (Fig. 70, B), and give rise in due course to the pronuclei of distinct gametes. 

 Forms in which precocious association of this kind occurs are described as 

 being " neogamous " (Woodcock). 



In many cases, union of distinct individuals can be observed which have 

 nothing to do with syngamy, since no fusion takes place of nuclei, but only 



