CHAPTER XII 



DEVELOPMENT OF RANA (THE FROG) 



Fertilisation. — The egg contains a large quantity of yolk, which 

 is aggregated at the vegetative pole. This pole is light in 

 colour when seen from outside, whereas the opposite animal 

 pole, and indeed the whole animal hemisphere, is darkly 

 pigmented. The nucleus is near the animal pole, which is 

 determined in the ovary, probably by the relation of the 

 developing egg to the little arteries and veins. 



The egg is surrounded by three membranes. The inner 

 vitelline membrane is secreted by the egg itself. Outside this 

 is a tough membrane formed by the follicle-cells which surround 

 the egg in the ovary. Outside this again is a coating layer of 

 jelly which is secreted by the glands of the wall of the oviduct, 

 as the egg passes down the latter on its way to the exterior. 



At the time of spawning, the males climb on to the backs of 

 the females, and as the latter extrude the eggs from their 

 cloacal apertures, the former shed the sperm over them. 

 Fertilisation thus takes place in the water outside the bodies of 

 the animals. One polar body has been extruded before the 

 egg is laid, the second polar body is pushed out after penetra- 

 tion of the sperm, and the egg- and sperm-pronuclei then fuse. 



The jelly swells out on contact with the water, and after 

 fertilisation the vitelline membrane becomes lifted off from the 

 surface of the egg. The egg is then able to rotate, and comes 

 to rest with the axis vertical, i.e. the vegetative pole with the 

 heavy yolk is turned downwards. 



The point of entrance of the sperm determines the median 

 plane of symmetry of the future embryo, and, soon after fertili- 

 sation, this is indicated by the formation of the grey crescent 



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