168 CELL DIVISION : DEVELOPMENT OF GERM-CELLS 



II. Development of the Germ-Cells. 



Iii the body of all multi- cellular animals there are produced 

 cells which have for their function the continuance of the species. 

 These cells, known as germ-cells or gametes, are of two kinds : 

 spermatozoa or male gametes, and ova or female gametes. As 

 a general rule ova cannot give rise to, new individuals until 

 they have been fertilised by spermatozoa ; and this process of 

 fertilisation again is only possible after both gametes have 

 attained maturity. 



A mature ovum consists of a usually spherical mass of cyto- 

 plasm enclosing a nucleus. The cytoplasm generally possesses 

 a sponge-like consistency, and contains in its meshes food 

 material and frequently pigment. The whole ovum is enclosed 

 in an elastic membrane, the vitelline membrane. 



Examine the ova of the earthworm or sea-urchin. Also draw 

 sections of the ovary of queen wasp, frog, or rabbit. 



A mature spermatozoon is always exceedingly small and 

 usually slender. It consists, in most animals, of a minute knob 

 or rod carrying a delicate whip-like filament. During life this 

 " tail " is capable of rapid vibratile movement, the tendency of 

 which is to cause the spermatozoon to swim with the knob 

 forwards. Hence this thickened end is called the " head." 

 Between the head and the whip-like tail is a minute thickening 

 the middle piece which is the root of the tail. 



Examine mature spermatozoa (from the vesicula seminalis of a 

 frog] in salt solution. Draw under the high power and notice the 

 movements due to flexion of the tail. 



Maturation of the Germ-Cells. Reduction of the 

 Chromosomes. 



A. Oogenesis. 



The primary ova are found at a very early stage of 

 animal development, in many cases before birth, and in a few 

 cases from the first cleavage of the egg into two cells, one of 

 which gives rise to the body and the other to the germ-cells. 



The primary ova are found in the peritoneum along each 



