4 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



to the isolecithal condition : but as a rule in the Vertebrate the 

 yolk is large in amount and is concentrated towards the lower or 

 abapical pole of the egg, the protoplasm towards the upper or apical 

 pole being comparatively poor in yolk (Telolecithal condition). 



This segregation of the dead yolk and the living protoplasm 

 towards opposite poles of the egg is well seen in the relatively huge 

 egg of the bird where the protoplasm is concentrated in a germinal 

 disc containing practically no yolk and forming a cap at the apical 

 pole of an enormous mass of yolk practically free from protoplasm. 



It has already been indicated that the egg may have a character- 

 istic coloration due to the colour of the yolk. Such yolk coloration 

 may be looked upon as accidental and without any special biological 

 significance in itself. Many eggs on the other hand especially 

 amongst the Ganoid fishes and the Amphibians are given a dark 

 colour by the presence within them of brownish-black pigments 

 belonging to the melanin group. Such pigment appears to be of 

 definite biological significance, providing as it does an opaque coat 

 which protects the living protoplasm from the harmful influence of 

 light. Eggs in which it occurs develop, as a rule, under conditions 

 where they are exposed to intense daylight. The eggs of ordinary 

 Frogs and Toads for example which are surrounded by clear trans- 

 parent jelly have a well-developed pigment coat. On the other hand 

 in the case of Frogs and Toads whose eggs are surrounded by light- 

 proof foam (see Chapter VIII.) or are deposited in burrows under- 

 ground they are commonly without pigment. 



In all probability this deposition of melanin pigment in the 

 superficial protoplasm of the egg (normally in its upper portion) is 

 to be interpreted as having been originally a direct reaction to the 

 influence of light, the metabolism being so affected as to bring about 

 the formation of this particular iron-containing excretory pigment. 



It may be objected that the pigment is produced before the egg 

 is laid (e.g. the Common Frog) and therefore before it is exposed to the 

 action of light, but as a matter of fact the body-wall of the adult is by 

 no means opaque to light rays and even while still in the ovary the 

 eggs are exposed to the influence of faint light. If we may take 

 it, as seems probable, that the influence of natural selection has 

 gradually developed in such cases the particular type of sensitiveness 

 to light which leads to the formation of melanin, on account of its 

 protective value, then there is nothing surprising in the developing 

 of this sensitiveness at earlier aud earlier peripds until at last it has 

 resulted in the pigmentation of the still intra-ovarian egg in response 

 to the feeble light rays which penetrate the body-wall. 



The other possible explanation of this precocious pigment forma- 

 tion is that the production of the pigment though originally taking 

 place as a direct reaction to light in the laid egg, has become so 

 engrained in the constitution of the species that it now comes about 

 even in the absence of the original stimulus. The objection to this 

 explanation is that it postulates the inheritance of an "acquired 



