IIATUBATION AND FEETILISATION OF THE EGG. 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 

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

 latter if the eggs are rotated. 



3. Laying of the Eg gs. 



The eggs when ripe are d ischarged from the ovary, and fall 

 in to the body cav ity ; along this th ey pass forwar Ss, directed 

 partly by con traction of the muscular bodv w alls, p artly by th e 

 ac tion of the cilia of the peritoneum , to th g mouths of the o vi- 

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

 b ody c avity, op posite the roots of the lung s. Within the first or 

 thick-walled part of the oviduct the eggs acquire gelatinous 

 investments secreted by glands in its walls : J;he ter minal part 

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

 tension, withm 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. Fertilisatio n of the Egg. 



The spermatozoa, after being shed over the spawn by the 

 male frog, swim actively by means of their longTaili", work their 

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

 vitelline membranes, and so penetrate into the eggs themselves, 

 wh ich they enter at, or clo se 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, D, Um), is the nucleus of the sperma- 



H 2 



