110 F. M. BALFOUR. 



brane. 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. 1). The germinal spot forms a nearly homo- 

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

 often eccentric position within the germinal vesicle, and is usually 

 rendered very conspicuous by its high refrangibility. In many 

 instances it has been shown to be capable of amoeboid move- 

 ments (Auerbach, and Os. Ilertwig), and is moreovej 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 investi- 

 gated with reference to the changes of germinal vesicle are those 

 with a single germinal spot, and it is with these that 1 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 ; — (1) 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 : (1) By one set 

 the germinal vesicle is stated to completely disappear and not to 

 be genetically connected with the subsequent nuclei of the 

 embryo. (2) According to the other set it remains in ovum 

 and by successive divisions forms the parent nucleus of all the 

 nuclei in the body of the embryo. Though the second of these 

 views has been supported by several very distinguished names the 

 first view was without doubt the one most generally entertained, 

 and Ilaeckcl (though from his own observations he was originally 

 a supporter of the second view) has even enunciated the theory 

 that there exists an anuclear stage, after the disappearance of the 

 germinal vesicle, which he regards as an embryonic repetition of 

 the monad condition of the Protozoa. 



While the supporters of the first view agree as to the disap- 



