INTRODUCTION 3 



the germinal vesicle, and one or more germinal spots. The outer 

 covering of the ovum is spoken of as the vitelline membrane. 



Since the ovum in its primitive form as above described repre- 

 sents a single cell, we may speak of the vitellus ^ as the protoplasm 

 of the egg-cell, the germinal vesicle as its nucleus, and the germinal 

 spot as its nucleolus. The cell-nucleus is enclosed by a delicate 

 nuclear membrane, and is made up of two constituents — the 

 spongioplasm or chromatin, and the hi/aloplasm or achromatin. 

 One or two small particles, the centrosomes, are also present in 

 the cell-body, and take an important part in the process of cell- 

 division. An outer limiting membrane, corresponding to the 

 vitelline membrane, is not an integral 

 part of the cell, but may be dilieren- 

 tiated as a hardening of the peripheral 

 protoplasm. 



In sexual reproduction, such as / I ^ J—\-iLji 



occurs in all Vertebrates, the fusion of -»- 

 the sperm-cell, containing the genera- 

 tive substance of the male, with the 

 ovum, is an absolute necessity for the 

 development of the latter. Fic. i.— Diagram of thk 



But before this can occur, certain Unimpregsatbd Ovum. 



changes take place in the ovum, which j)^ vitellus ; KB, germinal 

 are known as maturation. This con- vesicle ; KF, germinal spot, 

 sists of a twice-repeated process of cell- 

 division (Jcaryokinesis) similar to that which occurs in tissue- 

 cells, except that the resulting daughter-cells are of different 

 sizes, two small nucleated polar-cells (Fig. 2) being successively 

 thrown off from the larger ovum, the portion of the original 

 nucleus remaining in the ovum being known as the "■female 

 promicleus" A sperm-cell {spermatozoon) then makes its way into 

 the ovum, and its nucleus (the male pronucleus) unites with the 

 female pronucleus to form the segmentation nucleus. This 

 process, which is known as impregnation or fertilisation, thus 

 consists in a material fusion of the generative s^iistances of both sexes, 

 or more accurately oi'the sperm-nucleus and egg-nucleus. The essential 

 cause of inheritance can thus be traced to the molecular structure of 

 the nuclei of both male and female germinal cells. This structure 

 is the morphological expression of the characters of the species. 



After fertilisation has taken place development begins. The 

 segmentation nucleus divides into two equal parts, each of which 

 forms a new centre for the division of the oosperm, as it must 

 now be called, into two halves or blastomeres. This division, the 

 beginning of the process of segmentation, takes place by the 

 formation of a furrow round the egg which becomes deeper and 

 deeper until the division is complete. (Fig. 2, a). 



^ The vitellus consists of two different substances — protopla.sni ami daiUro- 

 plasm iyolJc) — in varying proportions in different animals. 



