I EMBRYOLOGY OF THE LOWER VERTEBRATES CH. 



t" tlit' isolecithal erudition : but us a rule in the Vertebrate the 

 yolk is large in amount and is concentrate 1 towards the lower or 

 abapical pole df tin- egg. the protoplasm towards tin- upper or apical 

 p<>le being comparatively poor in y,lk Telolecithal condition). 



This segregation of the dead yolk and the living protoplasm 

 towards opposite poles of tin- 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 charact.-r- 

 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 Aniphibians 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 formal ion of this particular iron-containing excretory pigment. 



It may be objected that the t pigrneiit is produced before the egg 

 is laid (e.ij. tin- < 'ommon Frog) and therefore before if 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 



exposed to tin- influence of faint light. If \\r may take 



ma prol.ahlc, that the inlhieiiee of natural selection has 



gradually developed in such eases the particular type of sensit iveiiess 



to light \\hich lei. Is to the formation of melanin, on account of its 



protective Value, then there is nothing surprising in the developing 



of th 't earlier aud earlier periods until at laM it has 



r. -ulted in i in- pigmentation of the still intra-ovarian egg in response 



fo the I'eehle light Hi \ - \\llicll penetrate I lie hod\-Ua!l. 



The other possible explanation of this precocious pigment forma- 

 tion is that the product inn of tin- pigment t hough originally taking 

 reaction to light in the laid egg, ha< become so 

 ,IIIM| in t he const it ut ion of the Specie* that it mm ( nines about 



ffl the i Of the in ' lllllllllS. The object foil to this 



nation i- that H populates I lie inheritance .if an 'a 



