YOLK 



animal, during the early stages of its development, from the need of 

 having to fend for itself. And, correlated with this, the necessity 

 of developing more or less 

 complicated adaptive fea- 

 tures to fit it for survival 

 as an independent free- 

 living creature in these 

 early stages is avoided. 



The yolk consists occa- 

 sionally of fluid but more 

 usually of rounded or 

 sub - angular granules of 

 highly nutritious material. 

 The yolk granules are 

 frequently of a character- 

 istic colour, yellow or 

 salmon colour or greenish, 

 and these impart their 

 colour to the egg as a whole. 

 Where however the yolk 

 becomes very finely subdi- 

 vided we find, as in the case 



P 



-*"«*—.. 



B 





Fig. 1. 

 A. — Section through egg of Amphioxus, 

 After Cerfontaine, 1906.) 

 The small crosses mark the position of apical and abapical 

 of Coloured 2"laSS 2T0Und P oles * Nea r the apical pole is seen the second polar body 

 . , -, ,? , , , ^ -, adherent to the surface of the egg. The egg- and sperm- 



lntO powder, tn&u tne Cnar- nuo lei have not yet fused, and are seen in close proximityito 



acteristic colour is replaced °ne another, 

 by white. This fine 

 subdivision of the 

 yolk with its accom- 

 panyingTvhite colour 

 is commonly found 

 in parts of the egg 

 where metabolism is 

 particularly active, 

 for example those 

 portions in which 

 active growth or cell 

 division is about to 

 take place, the fine 

 subdivision making 

 the yolk readily 

 assimilable and so 

 available for meta- 

 bolic needs. Where 

 the yolk is com- 

 paratively small in 

 amount, as in Am- 

 phioxus (Fig. 1, A), it may be distributed nearly equally through- 

 out the egg substance; in other words there is an approach 



B. — Section through egg of Polypterus, showing more marked 

 tendency for the yolk granules to concentrate towards 

 the abapical pole (TelolecitLal condition). 



n, nucleus. p, pigment. 



The yolk granules are indicated in both figures by dark dots. 



