MITOSIS IN GERM CELLS 117 



sex, but further changes may lead to the formation of ova in 

 the female or spermatozoa in a male frog. Whilst the frog is 

 still in the tadpole stage, certain of the epithelial cells of the 

 genital ridge become larger and more rounded than their 

 neighbours, and form definite germ-cells or primitive ova. 

 Around these the adjacent epithelial cells are arranged so as 

 to form a number of capsules or follicles which form slight 

 projections from the surface of the genital ridge. The 

 germinal epithelium gives rise to new primitive ova, or they 

 may be formed by division of those already existing. The 

 so-called primitive ova may give rise either to ova or sper- 

 matozoa, and the sexual differentiation only appears at about 

 the time of the metamorphosis. In the female frog the 

 process of differentiation is comparatively simple. The 

 primitive germ cells, which by continued multiplication by 

 division have become very numerous, enter into a period of 

 rest and growth, during which they grow much larger and 

 become filled with a number of granules of nutritive reserve 

 material known as the deutoplasm or food-yolk. The per- 

 manent ova are enclosed in follicles like the primitive ova, 

 and it is by the activity of the follicular cells that the food- 

 yolk is elaborated from the blood and lymph and passed on 

 to the ovum. The nucleus grows much larger, and its nuclear 

 membrane and chromatic reticulum become more distinct. 

 When the ovum has attained a size of about '5 mm., a thin 

 structureless envelope, the vitelline membrane, is formed around 

 it, and rather later black pigment is deposited at first over the 

 whole surface, but later it is restricted to one hemisphere. By 

 continued deposition of yolk within their cell-bodies the ova 

 grow till they reach the size of small shot, and then they 

 project in bunches from the surface. They then go through 

 a process known as maturation, during which each ovum 

 divides twice into two very unequal portions, the nucleus being 

 divided at "each division, but in a manner different to that 

 which obtains in tissue - cells. The two smaller portions 

 formed by division of the ovum are known as polar bodies. 

 After maturation the ova are discharged into the body-cavity 

 by the bursting of their follicles ; they are passed along partly 

 by muscular contractions of the abdominal walls, partly by 

 ciliary action, to the mouths of the oviducts, and are passed 

 down the oviducts to the uteri. During their passage down 



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