40 REPRODUCTION AND DEVELOPMENT. 



going further development until the male element, or sperm-cell, 

 is added to it. This process is called the fertilization, impreg- 

 nation, or fecundation of the ovum. During or after the proc- 

 ess of maturation the spermatozoon finds its way to the surface 

 of the ovule, passes through the vitelline membrane, or through 

 a small opening in it called the micropyle, and enters the proto- 

 plasm, where it forms a nucleus, the male pronudeus (Fig. 10). 

 The male and female pronuclei then coalesce, or at least be- 

 come apposed, and temporarily disappear, reappearing when 

 the cell undergoes division. The ovum is now fully formed 

 and capable of further development. It is a typical cell, 

 endowed potentially with all the attributes of vitality, and 

 contains within its minute compass almost inconceivable pow- 

 ers of definite and intricate expansion and evolution. It 

 might seem at first view that the largest part of the substance 

 of the fertilized ovum was contributed by the female parent, 

 while the contribution of the male was comparatively insig- 

 nificant ; but of the really vital substance, the chromatin, the 

 male element furnishes as much as the female, and hence makes 

 an equal impression on the offspring. To explain the ina- 

 bility of the unimpregnated female ovule to undergo division, 

 it has been suggested that it has no centrosome, and that this 

 is supplied by the spermatozoon, the addition of which there- 

 fore supplies all the conditions necessary for division. 



Development of the embryo : The further development of 

 the embryo proceeds by mitotic cell-division. The ovum 

 divides into two cells ; each of these into two more, and so 

 on. Thus a mass of cells is formed, constituting the blasto- 

 derm. Soon the cells of the blastoderm develop so as to 

 form two layers, the outer being called the epiblast, the in- 

 ner the hypoblast. Later another layer of cells appears 

 between these two, and is called the mesoblast. Up to a cer- 

 tain point the cells produced by repeated cleavage are similar 

 to one another ; but afterward differences of form and purpose 

 appear in different situations according to the organs and tis- 

 sues into which they are about to develop. This development 

 of cells having similar origins into dissimilar cells is the proc- 

 ess of differentiation. By the continued production and dif- 

 ferentiation of the cells of the epiblast, hypoblast, and meso- 

 blast, the embryo develops, and the various organs gradually 



