38 REPRODUCTION AND DEVELOPMENT. 



of the cell-body occurs ; the body has become indented, a line 

 of cleavage has appeared across it, and it divides entirely into 

 two parts. Thus the original cell has divided into two cells, 

 each like the parent in all respects. 



Conjugation : In contrast to the division of one cell into two, 

 the contrary process occurs, namely, the coalescence of two 

 cells to form one. This is called conjugation. It is observed 

 among some of the protozoa, two of these organisms becoming 

 completely fused to form one individual. A form of conju- 

 gation also occurs throughout the greater part of the entire 

 animal and plant kingdoms in connection with sexual repro- 

 duction. The fusion of the female and male gerrn-cells, the 

 ovum and spermatozoon in animals, the ovule and pollen in 

 plants, is a case of conjugation, although here the uniting cells 

 are somewhat different from each other. This form of conjuga- 

 tion is evidently of profound significance in the vital economy, 

 though not well understood. Even in the protozoa conjuga- 

 tion is perhaps connected with reproduction. 



Sexual reproduction : Multicellular animals are developed 

 from a female and a male cell, which coalesce and form a 

 single cell, the fecundated ovum. The spores from which 

 cryptogamous plants develop are likewise fertilized single 

 cells. Phenogamous plants are developed in essentially the 

 same way ; the fully formed seeds are, however, multicellu- 

 lar, though they can be traced back to a single fertilized ovule- 

 cell. The female element before fecundation is termed the 

 ovule; after union with the male element it becomes the ovum. 

 The male elements in animals are the spermatozoa ; in flow- 

 ering plants, the pollen. 



The ovule : The mammalian ovule, as fully developed in 

 the female ovary, is a comparatively large spherical cell about 

 0.2 millimetre in diameter. It is surrounded by a delicate 

 membranous wall, the vitelline membrane. The body of the 

 cell consists of protoplasm infiltrated with a large number of 

 minute fat-globules, for the early nourishment of the develop- 

 ing embryo ; this is the vitellus, or yolk. Within the vitellus, 

 located excentrically, is the nucleus of the cell, or the germi- 

 nal vesicle ; within this, again, is the nucleolus, or germinal 

 spot. 



Maturation of the ovule : Before or while the ovule receives 



