THE OVUM. 



19 



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 Chsetopoda 

 the components of the germinal spot become separated into two 

 nearly spherical masses (tig. 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 their character 

 and origin, and may be conveniently (Ludwig. No. 4) divided into 

 two great groups, viz. (1) those derived from the protoplasm of the 

 ovum itself or from its follicle, which may be called primary egg- 

 membranes; and (2) those formed by the wall of the oviduct or 

 otherwise, such as the egg-shell of a bird, 

 which may be called secondary egg- 

 membranes. 



The primary egg-membranes may 

 again be divided into two groups (Ed. 

 van Beneden, No. i), viz., (1) those 

 formed by the protoplasm of the ovum, 

 to which the name vitelline mem- 

 branes will be applied; and (2) those 

 formed by the cells of the follicle, to which 

 the name chorion will be applied. 



The secondary egg-membranes will be 

 dealt with in connection with the system- 

 atic account of the development of the 

 various groups. They coexist as a rule 

 with primary membranes, though in some 

 types (Cephalophorous Mollusca, many 

 Platyelminthes, etc.), they constitute the only protecting coverings of 

 the ovum. 



.if 



FIG. 6. OvrM OF To\-r>- 



PNEVSTES VAK1EUATUS WITH THE 

 PSEUDOPODIA-LIKE PROCESSES OF 

 THE PROTOPLASM PENETRATING THE 



ZONA RADIATA (z r). (After Se- 

 leuka.) 



