122 Elementary Zoology. 



derived from a single cell, just as we suppose that the ancestor, 

 of all the higher animals was a single-celled organism. More- 

 over, the eggs of many animals exhibit amoeboid movements ; 

 and the encystment previous to division of Protozoa, is 

 paralleled by the secretion of a membrane by the ovum when 

 it is ready to divide. 



Maturation of the Ovum. 



The egg, then, is a single nucleated cell, which only differs, 

 and that not always, from other animal cells by its greater size 

 and storage of yolk, the two characteristics being responsible 

 for each other. 



Except in the comparatively rare cases of parthenogenesis, 

 an ovum must be fertilised by a spermatozoon before it can 

 divide, and by its repeated divisions form an embryo. 



The hen's egg, on account of its large size and the dif- 

 ficulty of manipulating the yolk, has not been thoroughly 

 studied in the stages of its development preceding fertilisation. 

 The following account is therefore a general account, which, 

 with slight differences in detail, will no doubt serve as a de- 

 scription of the processes that occur in the fowl's egg. 



The nucleus is a vesicular structure surrounded by a definite 

 membrane, and in which there is a network of denser and 

 darkly staining matter — on this account termed chromatin — in 

 the meshes of which is a more fluid substance. The general 

 term " chromatin " has been given to the meshwork in general, 

 but it appears that the meshwork is formed of a groundwork of 

 linin, to which are adherent granules of nicdein. It is im- 

 portant to distinguish between these two substances, because 

 their behaviour is different, as will be seen shortly, in the 

 maturing and dividing ovum. Just before the ovum is ready 

 for fertilisation the nuclein masses itself into a number of short 

 rods, to which the name of chromosomes has been given. The 

 linin arranges itself in the characteristic form of a spindle, as 

 is shown in the annexed figure ; the membrane of the nucleus 

 has in the mean time disappeared, and at the two poles of the 



