GENERATION AND DEVELOPMENT 703 



extrusion, and in such animals artificial insemination is therefore 

 possible. The period of cestrus is not necessarily identical with 

 the period of ovulation ; cestrus may occur without ovulation, 

 and ovulation may occur without cestrus. Ewart says the mare 

 may mature and discharge one or more eggs after she has become 

 impregnated. Ovulation occurs at the moment the Graafian 

 vesicle ruptures and the ovum is ejected. The number of ova 

 which may be extruded during one sexual period is not known 

 with any degree of certainty ; in the case of the cat and dog there 

 is evidence of several being ejected, for each foetus represents a 

 separate egg. The number of eggs laid is always greatly in 

 excess of the number impregnated, and the mare, which probably 

 only produces one egg at a time, and with whom twin births are 

 very rare, is believed by Ewart to shed about ten or twenty ova 

 annually. Whether an equal number is discharged by each 

 ovary is unknown. Probably one ovum for the mare, cow, ass, 

 deer, elephant, or monkey, at each cestrous period is the rule, 

 though two may be discharged. The sheep probably discharges 

 one to four ; the dog, wolf, and cat five to six ; the pig ten. or 

 even fifteen. 



Fertilisation of the Ovurn. — Somewhere in the Fallopian tube, 

 though occasionally on the surface of the ovary, the meeting 

 between the sperm and germ cell occurs. The wriggling move- 

 ments of the sperm cell in the Fallopian tubes propel the sperma- 

 tozoa towards the ovary. The lashing of the cilia in the tube 

 assists the passage of the ovum towards the uterus. There is 

 no explanation of how the spermatozoa find their way into 

 those minute pin-point holes which represent the uterine end 

 of the Fallopian tubes. The spermatozoa meet the egg ; owing 

 to the enormous number existing even in a droplet of the 

 secretion, it is easy to understand why the ovum cannot escape 

 coming in contact with them. A single spermatozoon penetrates 

 the wall of the ovum, and enters the cytoplasm. Others may 

 attempt to follow, but the surface of the egg at once becomes 

 impervious to further attacks. The spermatozoon having got 

 inside the egg, and lying in its cytoplasm, loses its tail, which 

 is no longer required. The essential portion of the organism is 

 the head and middle piece. The former is the cell with its 

 nucleus, which contains the all-important chromosomes ; the 

 middle piece contains the centrosome, which, it will be remem- 

 bered, was lost to the mature ovum. There are now two nuclei 

 within the egg, one the male, the other the female pronucleus. 

 These meet, fuse, and, under the direction of the centrosome, 

 the process of cell division immediately follows their fusion. The 

 union of these two nuclei is a matter of extraordinary importance. 

 It will be remembered that by a process of reduction the 



