Ill SYNGAMY 73 



The case illustrated in the diagram is one where the oocyte I. proceeds 

 with the usual course of meiosis up to the end of the growth period, and 

 then pauses with its nucleus either in the " germinal vesicle " stage, or in 

 diakinesis, for the first meiotic division. The subsequent completion of 

 meiosis is dependent upon the entry of a spermatozoon into the oocyte. 

 Should this not occur, the oocyte perishes without further development. 

 The entry of a spermatozoon, however, immediately sets the meiotic pro- 

 cesses in operation again, and the nucleus moves towards the surface of 

 the egg, undergoes its two meiotic divisions to produce the first and second 

 polar bodies, and then returns to the centre of the egg to meet the male 

 nucleus. Examples of eggs in which fertiUzation is of this type are Asterias 

 (many workers) and Fasciola (Schellenberg, 191 1). Rarely the sperma- 

 tozoon enters the oocyte L at a much earUer stage, e.g. in Saccocirrus 

 (Buchner, 1914), where it enters at the beginning of the growth period 

 and Hes among the yolk during the whole of the long time which elapses 

 before the meiotic divisions of the egg take place and consequently 

 karyogamy becomes possible. Much more often, however, the sperma- 

 tozoon enters at a later stage. For instance, the pause in the meiotic 

 phase for the entry of the spermatozoon takes place in metaphase I. or 

 anaphase I. in Ophryotrocha (Korschelt, 1895), Chaetopterus (Mead, 1898 ; 

 Fig. 33), Thalassema (Griffin, 1899), Physa (Kostanecki and Wierzejski, 

 1896). It occurs in metaphase II. or anaphase II. in Amphioxus (Sobotta, 

 1897), the axolotl (Fick, 1893), and in the mouse and guinea-pig (Lams and 

 Doorme, 1908). In other cases the egg completes the process of meiosis, 

 forming both polar bodies, and the mature egg nucleus passes into the 

 resting condition to await the entry of the spermatozoon. Examples 

 of this type are the sea-urchins (many workers), and the sponge Syco?i 

 (Jorgensen, 1910 a). 



As regards the fusion of the two gametic nuclei, the diagram shows 

 each nucleus in the resting condition at the moment that karyogamy takes 

 place. In certain cases, however, the two nuclei do not fuse to form a 

 resting zygote nucleus, but they may fuse when in prophase for the first 

 cleavage division. In other cases the nuclei do not fuse before the 

 first cleavage mitosis at all, but each gamete nucleus forms its chromo- 

 somes while still independent, and the chromosomes are placed on the 

 spindle in two separate groups. In these cases the chromosomes derived 

 from the male and female parents are not united into a zygote nucleus 

 until the telophase of the first cleavage mitosis. Examples of karyogamy 

 of this type are Ascaris megalocephala (cf. Fig. 70) and Ophryotrocha 

 (Fig. 34). Indeed, it not infrequently happens that the complete fusion 

 of the gamete nuclei is postponed to a much later stage (p. 77). 



The male nucleus, which is derived from the head of the spermatozoon, 

 is at first very much smaller than the female. This is obviously due 



