THE EGG 25 



The Animal Egg. 



The mature egg, capable of fertilisation, is everywhere in the 

 animal kingdom a simple cell, and shows the typical structure of such 

 a cell. It consists of protoplasm, called yolk, and of a nucleus which 

 is named the germinal vesicle in an unfertilised egg. The egg 

 is either naked or surrounded by one or more membranes and 

 envelopes. These are of very different nature according to their 

 origin. They are either secreted by the egg cell itself, and then 

 answer, as real primary yolk or egg integuments, to the membranes 

 of ordinary cells, or they are in various ways supplied by the 

 surrounding tissues of the mother body, and are laid round the egg 

 externally. In this case they may be considered as secondary or 

 accessory egg envelopes. The eggs arise in special organs of the 

 Metazoan body, called ovaria or germaria. These, in the simplest 

 cases, are masses of cells, some of which by stronger growth become 

 egg cells. 



The processes of the formation, growth, and ripening of the egg in 

 the animal kingdom are as various as is the structure of the ovary 

 itself. It is especially the necessity for the abundant nourishment of 

 the eggs which determines the most manifold modifications. The eggs 

 are developed either from a mass of protoplasm with nuclei scattered in 

 it, or from an assemblage of similar little cells clearly defined one from 

 the other. In the first case the nuclei, in the second the cells, multiply 

 by fission. After this multiplication has lasted for some time the proto- 

 plasm round the nuclei in the first case separates off and gives rise 

 here also to independent cells. All these young cells are young egg 

 germs, and capable of growing and ripening into eggs ; but only in the 

 rarest cases do all the cells become eggs a large number of germ cells 

 have almost always another fate. 



The nourishment of the egg, speaking generally, is secured in the 

 following ways. In animals without a blood -vascular system and 

 body cavity the ovaries lie on the wall of the intestine, or of a 

 gastro- vascular system, which proceeds from the main intestine 

 (Coslenterata, Platodes). In many animals the eggs are developed on 

 the wall of the body cavity and nourished by the body fluids, 

 into which they sooner or later pass and mature while suspended 

 in them (many higher worms). In the case of the greater number 

 of the higher animals the nourishment of the egg is secured by a 

 rich provision of blood-vessels in the ovaries. In those cases 

 where only some of the egg germs are developed into eggs, 

 the remainder often serve as nourishment for these or contribute 

 to their nutrition. Amoeboid moving egg cells can feed upon the 

 neighbouring egg germs after the manner of Amcebce ; or the surround- 

 ing egg germs store up food, which they give over to the growing 

 egg cell, either by emptying their contents into it (Cephalopoda) 



