20 EMBRYOLOGY OF THE LOWER VERTEBRATES oh. 



which produce eggs of very large size e.g. Gymnarchus niloticus 

 (Budgett, 1901 ; Assheton, 1907) where they measure about 10 mm. 

 in diameter, or the Salmon or Trout where they measure from 

 4 to 5 mm. 



The majority of fishes produce eggs in enormous numbers, 

 amounting in some cases to several millions, and correlated with 

 this the size of the individual egg has become much reduced. The 

 average diameter of a Teleostean egg may be taken as about 1 mm. 

 In an egg of this size segmentation of so markedly meroblastic a 

 character would be puzzling except on the hypothesis that the 

 meroblastic condition had arisen in ancestral forms in which the 

 eggs were much larger. 



The larger part of the egg consists of a spherical mass of 

 practically pure yolk. On the surface of this is a thin layer of 

 protoplasm containing droplets of oil, and this layer of protoplasm 

 is more or less distinctly thickened in the region of the apical pole 

 to form a germinal disc in which is contained the nucleus. Irregular 

 prolongations of the superficial protoplasm may sometimes, especially 

 in immature eggs, be traced inwards into the substance of the yolk. 



A characteristic feature of many teleosts is the tendency for the 

 yolk to, assume a liquid form. This is particularly marked in many 

 pelagic eggs where it is not merely liquefied but runs together at the 

 time of spawning or of fertilization to form a sphere of glassy 

 transparency. There may further be, interspersed amongst the 

 ordinary yolk, droplets of oily looking fluid often with a distinctive 

 colour. These may unite into a few droplets or into a single larger 

 drop forming a conspicuous, often coloured, sphere in the midst of 

 the ordinary yolk. The colour and size of such drops frequently 

 afford an easy means of recognizing the species to which a particular 

 egg belongs. They may also have a characteristic position and may 

 be surrounded by a special condensation of protoplasm or, on the 

 other hand, they may simply float freely in the main mass of fluid 

 yolk. Although these droplets may, as already indicated, exhibit 

 peculiarities characteristic of particular species they do not seem to 

 give indications of genetic affinity in regard to genera or larger 

 groups : nor do they show any definite relation to the conditions, 

 pelagic or otherwise, under which the egg develops (Prince, 1886). 



The yolk of teleosts is also characterized by a diminution of its 

 specific gravity which causes the egg to assume a reversed position 

 with the apical pole below, and which further, in the case of a vast 

 number of marine fishes, causes the egg as a whole to float freely 

 suspended in the sea water. 



Seeing that the Teleostei as a group is above all characterized by 

 specialization for a swimming existence, independent of a solid 

 substratum, we are perhaps justified in assuming that the freely 

 floating pelagic mode of development above mentioned was originally 

 present throughout the group. The demersal type of development, 

 where the eggs are deposited on the, solid substratum, would then 



