THE OVUM. 23 



forms a nearly homogeneous body, with frequently one or more 

 vacuoles. It often occupies an eccentric position within the 

 germinal vesicle, and is usually rendered very conspicuous by its 

 high refrangibility. In many instances it has been shewn to be 

 capable of amceboid movements (Hertwig, Eimer), and is more- 

 over more solid and more strongly tinged by colouring reagents 

 than the remaining constituents of the germinal vesicle. 



In many instances there is only one germinal spot, or else 

 one main spot and two or three accessory smaller spots. In 

 other cases, e.g. Osseous Fishes, Echinaster fallax, Eucope poly- 

 styla, there are a large number of nearly equal germinal spots 

 which appear to result from the division or endogenous prolifera- 

 tion of the original spot. Sometimes the germinal spots are 

 placed immediately within the membrane of the germinal vesicle 

 (Elasmobranchii and Sagitta). In many Lamellibranchiata, in 

 the earth-worm, and in many Cha^topoda the components of the 

 germinal spot become separated into two nearly spherical 

 masses (fig. 12), which remain in contiguity along a small part 

 of their circumference, and are firmly united together. The 

 smaller of the two parts is more highly refractive than the 

 larger. Hertwig has shewn that the germinal spot is often 

 composed of two constituents as in the above cases, but that the 

 more highly refractive material is generally completely enclosed 

 by the less dense substance. By Fol the germinal spot is stated 

 to be absent in a species of Sagitta, but this must be regarded 

 as doubtful. In young ova the relative size of the germinal 

 vesicle is very considerable. It occupies in the first instance a 

 central position in the ovum, but at maturity is almost always 

 found in close proximity to the surface. Its change of position 

 in a large number of instances is accomplished during the 

 growth of the ovum in the ovary, but in other cases does not 

 take place till the ovum has been laid. 



As the ovum attains maturity, important changes take place 

 in the constitution of the germinal vesicle, which are described 

 in the next chapter. 



The egg-membranes. A certain number of ova when 



ready to be fertilized are naked cells devoid of any form of 



protecting covering, but as a rule the ovum is invested by some 



.form of membrane. Such coverings present great variety in 



