GERMINAL VESICLE AND FIRST EMBRYONIC NUCLEUS. 167 



size. It is circular, and its contour is quite regular. Some- 

 times it appears nodulated on its surface ; and lastly, 

 at other times it is angular and altogether irregular. 

 It is formed of a highly refractive and very brilliant sub- 

 stance which encloses clear vacuoles, whose number and 

 volume vary in different eggs. The vacuoles have already been 

 noticed by Leydig, in the eggsof Holothuria tubulosa: "Der 

 Keimfleck des fertigen Eies ist bedeutend scharfer con- 

 tourirt als das Keimblaschen, fast fettartig und zeigt ein oder 

 mehrere Cavitaten." Hertwig has observed that in the ova of 

 the Toxopneustes Wagner's spot contains sometimes one, 

 sometimes several vacuoles. 



In the ova which I am describing, I have not observed 

 amoeboid movements performed by the nucleolus. Long ago 

 de la Valette^ had noted the fact that in the ovum of a 

 Libellula the germinal spot might be seen to change its f-^rm 

 and place. Mecznikow^ has seen, not only in the spot of 

 Wagner of several of the lower animals, but also in the 

 salivary cells of the larva of insects, spontaneous movements 

 of the nucleoli. Balbiani^ has made a similar observation 

 with regard to the ova of spiders. Alexander Brandt"^ has 

 observed that in the eggs of Periplaneta orientalis the ger- 

 minal spot affects all sorts of forms, and that these changes, 

 which are really active, must be ascribed to the contractility 

 of the substance of the spot or nucleolus. He has remarked 

 that under the influence of heat, these movements become so 

 active, that it is difficult to draw the successive changes of 

 form which are produced. 



Auerbach^has recognised changes of form, presenting all 

 the characters of auiceboid movements, in the large nuclei of 

 the embryonic cells of the Muscida and in the germinal spots 

 of the ovum of the pike ; Hertwig has seen the same 

 phenomenon occur in the nuclei of the eggs of the frog and 

 also in the germinal spot of \N agner, in the ovum of 

 Pterotrachea. More than four years ago, I observed 

 changes in form, and enlargement and diminution of size, 

 whilst watching Wagner's spot in the germinal cells of the 

 Polystomum integerrimum, and I had remarked that the 

 diminution in volume of these nucleoli corresponded with 



' De la Valette : " Uber den Keimfleck und die Deutung der Eillieile," 

 ' Archiv. fiir Mikrosk. Auat./ bd. ii, 1866. 



^ Mecznikow, ' Virchow's Ajcbiv.,' bd. xii. 



2 Balbiani (quoted by Auerbach), see Keferstein. ' Jahresbericht,' fiir 

 1865. 



■• Alexander Brandt, " Uber retive Forraveranderungen des fCernkorper- 

 chens," ' Archiv. fiir Mikrosk. Anat.,' bd. ix. 



* L. Auerbach, ' Organologsclie Sludien,' heft i, pp. 167 and 168. 



