522 MATURATION AND IMPREGNATION OF THE OVUM. 



containing (3) one or more germinal spots or nucleoli. It is 

 with the germinal vesicle and its contents that we are especially 

 concerned. This body at its full development has a more 

 or less spherical shape, and is enveloped by a distinct membrane. 

 Its contents are for the most part fluid, but may be more or 

 less granular. Their most characteristic component is, however, a 

 protoplasmic network which stretches from the germinal spot to 

 the investing membrane, but is especially concentrated round 

 the former (Fig. i). The germinal spot forms a nearly homo- 

 geneous body, with frequently one or more vacuoles. It occupies 

 an often excentric 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 amoeboid 

 movements (Auerbach, and Os. Hertwig), and is moreover more 

 solid and more strongly tinged by colouring reagents than the 

 remaining constituents of the germinal vesicle. These peculiari- 

 ties have caused the matter of which it is composed to be 

 distinguished by Auerbach and Hertwig as nuclear substance. 



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

 main spot, and two or three accessory smaller spots. In other 

 cases, e.g. Osseous Fish, there are a large number of nearly equal 

 germinal spots. The eggs which have been most investigated 

 with reference to the changes of germinal vesicle are those with 

 a single germinal spot, and it is with these that I shall have more 

 especially to deal in the sequel. 



The germinal vesicle 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. 



The questions which many investigators have recently set 

 themselves to answer are the two following: (i) What becomes 

 of the germinal vesicle when the ovum is ready to be impregnated ? 

 (2) Is any part of it present in the ovum at the commencement 

 of segmentation ? According to their answers to these questions 

 the older embryologists roughly fall into two groups: (i) By 

 one set the germinal vesicle is stated to completely disappear 

 and not to be genetically connected with the subsequent nuclei 



