INTRODUCTION 3 



(Fig. I), consisting of the vitellus, in the interior of which is the 

 germinal vesicle, enclosing one or more germinal spots: the 

 membrane which covers the vitellus is spoken of as the vitelline 

 membrane. Since the ovum corresponds to 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 vitellus, however, consists of two different sub- 

 stances protoplasm and deuteroplasm (yolk} in varying propor- 

 tion and relative distribution in different animals. 



The nucleus is enclosed by a delicate nuclear membrane, and is 

 made up of two chief constituents the spongioplasm or chromatin, 

 and the hyaloplasm or achromatin. A 

 small particle, the ccntrosome, is also 

 present in the protoplasm of the cell, 

 and takes an important part in the 

 process of cell-division. An outer 

 limiting membrane, corresponding to 

 the vitelline membrane, is not an in- 

 tegral part of the cell, but may be 

 differentiated from the peripheral pro- 



toplasm ^ IC{ - 1- DIAGRAM OF THE 



In the process of sexual reproduc- 



tion, which occurs in all Vertebrates, A vitellus; KB, germinal 

 the fusion with the ovum of the vesicle ; KF > S erminal s P ot ' 

 sperm-cell or spermatozoon, con- 



taining the generative substance of the male, is an absolute 

 necessity for the development of the former. 



Before this fusion can occur, certain changes take place in 

 the ovum which constitute what is known as its maturation, the 

 essential result of which is a reduction in mass of the chromatin 

 in the germinal vesicle. The ovum undergoes a twice-repeated 

 process of cell division (karyokinesis or mitosis) similar to that 

 which occurs in tissue-cells, except that the resulting daughter- 

 cells, in addition to the reduction in their chromatin, are of 

 different sizes, two small evanescent polar-cells (Fig. 2) being suc- 

 cessively thrown off from the larger ovum. A similar process also 

 occurs in the development of the sperm-cell, except that there is 

 no difference in size between the products of division. The 

 portion of the original nucleus remaining in the ovum or sperm 

 is then known respectively as the female (or maternal) and male (or 

 paternal) pronucleus. 



A sperm-cell then makes its way into the ovum, and its pro- 

 nucleus unites with the female pronucleus to form the segmentation 

 nucleus. This process, which is known as impregnation or 

 fertilisation, thus consists in a material intermingling of the 

 generative substances of both sexes, or more accurately of the 

 sperm-nucleus and egg- nucleus. The essential cause of inherit- 

 ance can thus be traced to the molecular structure of the nuclei of 



B 2 



