MATURATION AND FERTILISATION OF THE EGO. 99 



The formation of the second polar body in the frog has not 



been seen, but there can be little doubt that it is due, as in 

 other animals, to a further division of the part of the nuclear 

 spindle which remains within the egg, after extrusion of the first 

 polar body. According to Schultze, the extrusion of the second 

 polar body from the egg does not take place until about half 

 an hour after fertilisation of the egg ; i.e. after the entrance of 

 the spermatozoon, but before the completion of the act of fer- 

 tilisation. 



The two polar bodies are of about equal size ; they lie freely 

 011 the yolk, in the peri-vitelline fluid, and shift about with this 

 latter if the eggs are rotated. 



3. Laying of the Eggs. 



The eggs when ripe are discharged from the ovary, and fall 

 into the body cavity ; along this they pass forwards, directed 

 partly by contraction of the muscular body walls, partly by the 

 action of the cilia of the peritoneum, to the mouths of the ovi- 

 ducts, which are situated at the extreme anterior end of the 

 body cavity, opposite the roots of the lungs. Within the first or 

 thick-walled part of the oviduct the eggs acquire gelatinous 

 investments secreted by glands in its walls : the terminal part 

 of each oviduct is a thin-walled pouch, capable of great dis- 

 tension, within which the eggs accumulate in large numbers. 

 Finally, at the time of copulation, the eggs are passed out 

 through the cloacal opening into water, in which the albuminous 

 investments of the eggs speedily swell up to form the gelatinous 

 mass of the frog's spawn. 



4. Fertilisation of the Egg. 



The spermatozoa, after being shed over the spawn by the 

 male frog, swim actively by means of their long tails, work their 

 way into the gelatinous mass of the spawn, bore through the 

 vitelline membranes, and so penetrate into the eggs themselves, 

 which they enter at, or close to, their upper or black poles. 



A single spermatozoon is sufficient to fertilise an egg, and it 

 is doubtful whether more than one is ever concerned in the pro- 

 cess. About an hour after the spermatozoon has entered, a pig- 

 mented process may be seen projecting into the egg from the 

 point of entry (Fig. 45, D), and in the centre of the process a clear 

 spot. This spot (Fig. 45, Df V*0< is the nucleus of tte: 



