CHAPTER XII 



EMBRYOLOGY 



Structure of the Egg. 



The egg, when laid, consists of the following parts : 



(i) The germ-vesicle, or nucleus of the mature germ-cell. This 

 is a large vesicular nucleus shewing a fine network of chromatin. 

 It is deeply embedded in the yolk, near the centre of the egg. 



(ii) A fine network of formative protoplasm, ramifying through 

 the yolk. 



(iii) The periplasm, a very fine layer of formative protoplasm 

 completely surrounding the yolk. 



(iv) The food-yolk, an immense quantity of homogeneous, 

 structureless yellow material, in the form of originally spherical 

 globules compressed by mutual pressure. Drops of fat are also 

 present. 



(v) The vitelline membrane, quite structureless, colourless and 

 exceedingly fine. It is secreted from the egg itself, and closely 

 invests the yolk-mass. 



(vi) The chorion, a tough outer covering, overlying (v), and 

 formed of chorionin. This is secreted by the follicle cells of the 

 ovary, which leave a more or less definite irregular hexagonal pattern 

 upon it (representing the boundaries of the follicle cells) except 

 at the two ends, where the same pattern falls upon the thickened 

 gelatinous envelope. At the anterior pole of the egg (i.e. the pole 

 of attachment to the egg-string), the chorion is strongly thickened. 

 A delicate canal, placed obliquely a little to one side of the central 

 axis, pierces the chorion at this end. This is the micropyle, or 

 passage for the entry of the sperm. The chorion is also slightly 

 thickened at the posterior pole, but in a less pointed fashion. 



(vii) An outer layer of gelatinous matter, serving to protect the 



