THE FROG 105 



enormous amount of food material, the yolk, in the form of small 

 spheres. It is half black and half white, the upper, dark, or animal 

 pole contains the nucleus with a certain amount of accompanying 

 cytoplasm, the white or vegetative pole a much greater proportion 

 of inert yolk. The egg when it leaves the ovary is enclosed in a 

 very thin almost structureless membrane, the vitelline membrane, 

 and as it passes down the oviduct it has added to this a thicker but 

 still relatively thin coat of a mucilaginous substance. The semen 

 or spermatic fluid contains in addition to albuminous substances a 

 countless number of microscopical cells, the sperms. They are also 

 single cells, and consist of a head of denser material, containing the 

 nucleus and a fine thread-like tail, by means of which they are able 

 to swim actively for a short time. When they come into contact 

 with an ovum they start swimming vigorously into it until one 

 penetrates the vitelline membrane, which then becomes impermeable 

 to any more. Once inside the membrane the nucleus from the head 

 enlarges and travels towards the nucleus of the ovum, and then the 

 two nuclei fuse together, this actually constituting fertilisation. 

 There is produced as the result of this process a large cell with but 

 a single nucleus, formed by the union of the male and female pro- 

 nuclei, as they are termed, and this constitutes the fertilised ovum 

 or oospore, the germ from which the new being will arise. It is 

 only after fertilisation that the egg can grow up into a frog, so that 

 we are here dealing with an extremely important phenomenon, and 

 it is also a significant fact that the product of fertilisation is a single 

 cell with a single nucleus. The importance of these things leads to 

 their frequent discussion and the use of a special terminology. When 

 we wish to speak of either a male or female reproductive organ 

 without discriminating between the sexes we use the term gonad, 

 and in the same way either ovum or sperm can be called a gamete. 

 The union of two gametes, i.e. fertilisation, produces a zygote. 



The egg when laid has a diameter of about 175 mm. Immedi- 

 ately after impregnation the mucous coat absorbs water and starts 

 to swell up rapidly, until it forms a relatively large transparent 

 sphere, in which lies the black and white fertilised ovum. Hundreds 

 of eggs are laid at the same time, and when they all swell they adhere 

 together to form the large frothy masses common in our ponds and 

 ditches in early spring, and known as frog's spawn. 



Soon after the above processes have taken place a groove appears 

 on the animal pole of the egg, and this spreads until it has encircled 

 the whole ovum. Examination of sections shows that this groove 

 passes right through the egg, and that the original nucleus has divided 

 .into two, one in each half, so that we now have two cells instead of 

 one. No sooner is this completed than another furrow at right angles 



