STRUCTURE OF THE EGG. 3 



and especially in fish, the two periods usually overlap each 

 other, the reproductive organs attaining maturity before growth 

 of the animal as a whole is completed ; but in some Invertebrates 

 the division becomes a very sharp one. The silkworm-moth and 

 the Ephemera or may-fly afford well-known instances of this ; 

 the greater part of the life cycle being spent as larvse, which 

 feed vigorously and grow rapidly, but are incapable of reproduc- 

 tion ; while the adult insects, the moth or the may-fly, are 

 capable of reproducing, but take no food and live but for a few 

 hours. 



Structure of the Egg. 



The egg is a single nucleated cell. In some Invertebrates, 

 as in Sagitta and in certain insects, the cells from which the 

 eggs arise may be distinguished at a very early stage in the 

 development of the embryo, or even from its actual commence- 

 ment ; but in most Invertebrates, and in all Vertebrates, the 

 somatic and reproductive cells are at first indistinguishable from 

 one another, and it is not until the embryo has advanced con- 

 siderably in its development that the reproductive cells can be 

 recognised as such. 



The eggs, or germ-cells, may be distributed over a consider- 

 able part of the body of the animal, as in the Nemertine worms, 

 in Balanoglossus, and, to a less extent, in Amphioxus ; more 

 usually, and constantly in the higher members of a group, they 

 are restricted to particular organs, the ovaries. In the early 

 stages of development, the reproductive organs usually extend 

 over a greater part of the length of the animal than they do 

 in the adult condition ; in the frog, for example, the ovaries 

 undergo during development an actual shortening or concen- 

 tration, a considerable part of their length degenerating. 



The egg or germ-cell, like any ordinary cell, consists of a 

 cell-body, containing a nucleus, and inclosed within an elastic 

 vitelline membrane. The egg is usually more or less spherical, 

 but may be irregular in shape, or even, as in Hydra, amoeboid. 

 The cell-body consists of protoplasm, in which a more or less 

 pronounced reticular structure is present ; the protoplasm form- 

 ing a network of firmer strands, the meshes of which are filled 

 with a more fluid substance, in which are contained minute 

 particles or granules in greater or less number. The nucleus, 



B 2 



