LESSON XXIII 



SPERMATOGENESIS AND OOGENESIS. THE MATURATION AND 

 IMPREGNATION OF THE OVUM. THE CONNECTION BE- 

 TWEEN UNICELLULAR AND DIPLOBLASTIC ANIMALS 



In the preceding lessons it has more than once been stated 

 that sperms arise from ordinary undifferentiated cells in the 

 spermary, and that ova are produced by the enlargement 

 of similar cells in the ovary. Fertilisation has also been de- 

 scribed as the conjugation or fusion of ovum and sperm. We 

 have now to consider in greater detail what is known as to 

 the precise mode of development of sperms {spermatogenesis) 

 and of ova {oogenesis), as well as the exact steps of the pro- 

 cess by which an oosperm or unicellular embryo is formed 

 by the union of the two sexual elements. The following 

 description applies to animals : recent researches show that 

 essentially similar processes take place in plants. 



Both ovary and spermary are at first composed of cells of 

 the ordinary kind, the primitive sex-cells^ and it is only by 

 the further development of these that the sex of the gonad 

 is determined. 



In the spermary the sex-cells (Fig.6o, a) undergo repeated 

 fission, forming what are known as the sperm-mother-cells 

 (b). These have been found in several instances to be 



