72 



CYTOLOGY 



CHAP. 



gamete. The general course of events is illustrated by the diagrammatic 

 Fig. 32 and is sufficiently explained by the legend of that figure. Fig. 

 33 on the other hand shows the process as it actually occurs in the AnneUd, 



p.hl 



Fig. 32. 



Diagram of fertilization. A, entry of spermatozoon into primary oocyte ; B, the ? nucleus has 

 moved to the surface and is in metaphase I. ; C, 9 nucleus in telophase I. The tail of the spermatozoon 

 has broken off and is degenerating. The head of the spermatozoon has rotated so that the two centrosomes 

 which have disengaged from the middle piece now precede the S nucleus as it travels towards the centre of 

 the egg. D, 9 nucleus in telophase II. ; first polar body dividing; E, approach of the S and 9 nuclei ; F, 

 the two nuclei, now of approximately equal size, and in early prophase, are in contact ; G, the two nuclei have 

 fused into a zygote nucleus : formation of spindle figvure out of centrosomes and asters introduced by the 

 spermatozoon ; H, telophase of first (cleavage) division of the zygote nucleus ; I, 2-celled stage. 



A,, head of spermatozoon {S nucleus); m, middle piece; p.b.i andp.b.2, first and second polar bodies; 

 t, tail of spermatozoon. 



Chaetopterus. The chief variations from the examples depicted concern 

 the moment of entry of the spennatozoon, and the condition of the 

 gametic nuclei (often called the male and female pronuclei) at the moment 

 of karyogamy or nuclear fusion. 



