14 



FEKTILISATION 



forms, in which certain individuals of the colony become macro-, others micro-gametes 

 (e.g. Volvox), and from this stage the step to the differentiation of dimorphic sex-cells is 

 theoretically a simple one. 



Fertilisation. The ovum after its expulsion from the Graafian follicle is 

 received upon the fimbriated end of the Fallopian tube. The fimbrise are covered 

 by a prolongation of the ciliated lining of the tube, and the action of the cilia 

 serves to propel the minute ovum into and along the tube towards the uterus. 

 In this passage it may, if impregnation has occurred, meet with the spermatozoa, 

 and one of them may penetrate the zona pellucida to fertilise the ovum. It is 

 possible in some instances for fertilisation to occur on the fimbriated extremity 



of the tube, or even in the 

 Graafian follicle, and this 

 may result in an extra - 

 uterine pregnancy. 



The details of the pro- 

 cess of fertilisation have 

 been observed in a few 

 mammals, most clearly 

 in the mouse by Sobotta. 

 The process can be easily 

 followed in the transparent 

 egg of an echinoderm, and 

 for this reason the pheno- 

 mena as seen in the com- 

 mon sea-urchin will first be 

 briefly described, and then 

 a comparison will be made 

 with the facts established 

 for the egg of the mouse. 

 This is the more convenient 

 because the two cases re- 

 present two types of the 

 process. 



When the spermato- 

 zoon which is to effect 

 the fertilisation of the 

 echinoderm - egg touches 

 its surface the protoplasm 

 streams out at the point of 

 contact, to form what is 

 known as the entrance cone 

 (fig. 23, 6). As soon as 

 the sperm-head is fixed, a membrane is thrown off from the egg. It may 

 happen, however, that before this is effected several spermatozoa may have 

 obtained an entrance, and polyspermy results. This is always followed by 

 abnormal development. The spermatozoon, by the action of the flagellum, 

 now bores its way into the egg until the whole head and the middle piece 

 are imbedded in the protoplasm. The flagellum, no longer of service, is 

 thrown off, and the sperm-head undergoes a rotation through 180 (fig. 23, 6, c), 

 until the middle piece is directed inwards. Radiations now appear in the 

 protoplasm centred on the situation of the middle piece, which is no longer 

 distinguishable, and the sperm-head commences a movement towards the 

 centre of the egg. In living eggs the radiations are seen gradually to extend 



PIG. 23. FERTILISATION IN ECHINUS ESCULENTUS ; DRAWN FROM 



SECTIONS HIGHLY MAGNIFIED. (T. H. Bryce.) 



a, entrance of spermatozoon ; b, commencing rotation of sperm- 

 head ; c, completed rotation of sperm-head ; commencing sperm- 

 aster. 



