4 INTRODUCTION. 



or germinal vesicle as it is often called, is large, sometimes as 

 much as half the diameter of the egg itself: it is usually placed 

 excentrically, and consists of an outer nuclear membrane, en- 

 closing a clear coagulable liquid, the nucleoplasm. Traversing 

 the nucleoplasm is a reticulum, formed of one or more com- 

 plexly coiled threads of a substance which, from the readiness 

 with which it absorbs colouring matters, is termed chromatin. 

 One or more nucleoli, or germinal spots, are very commonly 

 present as small, deeply-staining spherical bodies : they appear, 

 however, to be non-essential structures, and are in many cases 

 only nodes or local thickenings of the reticulum. It is stated by 

 some investigators that the nuclear membrane is not a continu- 

 ous structure, but is really a denser and more superficial part of 

 the nuclear reticulum ; and that the nuclear reticulum and the 

 reticulum of the protoplasmic cell-body are directly continuous 

 with each other. 



Food-Yolk. The meshes of the protoplasmic cell-body of the 

 egg always contain granules. These vary greatly in number 

 and in size in the eggs of different animals, and constitute a 

 store of nutrient matter, at the expense of which the develop- 

 ment of the egg and the formation of the embryo are effected ; 

 they may be spoken of collectively as deutoplasm or food- 

 yolk. 



These granules of food-yolk, though always present in greater 

 or less quantity, are to be regarded as accessory rather than as 

 essential parts of the egg. In every egg we must distinguish 

 between (1) the living protoplasm of the egg, out of which the 

 embryo is directly developed, and which may be spoken of as 

 germ-yolk ; and (2) the deutoplasm, or granules of food-yolk, 

 consisting of non-living particles of nutritious matter, imbedded 

 in the meshes of the protoplasm, which do not directly form any 

 part of the embryo, but which indirectly render its development 

 possible, by nourishing the active protoplasm. 



The relation between the protoplasm or germ-yolk and the 

 deutoplasm or food-yolk is, in fact, the same as that between 

 the traveller and the sandwiches or other provisions that he 

 carries with him to nourish him during his journey. The sand- 

 wiches are non-living ; but during the journey they are gradually 

 consumed, absorbed, and utilised for the nutrition of the living 

 tissues of the traveller. 



