TERMINAL VESICLE AND FIRST EMBRYONIC NUCLEUS. 157 



I. Does the germinal spot disappear or does it remain so 

 as to become the central pronucleus {Eikern of Hertwig) ? 



Hertwig expresses the opinion that the germinal vesicle 

 disappears in Toxopneustes lividus ; but he believes that the 

 germinal spot (body or spot of Wagner) remains so as to 

 become his (nucleus of the e^g). He has not been able to 

 arrive at a direct proof of this proposition ; his opinion rests 

 upon indirect proofs, of which I will give a summary. 



(1.) The nucleus of the e^^ has the same dimensions 

 (13 fx) as Wagner's germinal spot; both are corpuscles 

 ■without any membrane and formed of tolerably firm and 

 homogeneous substance. 



(2.) Like the substance of Wagner's spot, the nucleus of 

 the e^^ is coagulated by osmic acid and coloured more deeply 

 black than the yolk. Both are stained red by carmine. 

 When acted on by acetic acid and chromic acid the two 

 elements undergo a sort of superficial coagulation which 

 produces a finely granular cortical layer, and some spots 

 also granular, in the interior. 



(3.) We never observe the nucleus of the egg and the 

 germinal spot at the same moment in the same egg. So 

 long as the germinal spot is seen in the germinal vesicle, 

 when the latter has become superficial and lenticular, it is 

 impossible to discover a nucleus in the yolk. From the 

 time when this nucleus exists the germinal vesicle is destitute 

 of its spot. The two elements are never wanting at the same 

 time. 



(4.) Hertwig has never succeeded in tracing any alteration 

 in Wagner's spot, even in those eggs where the germinal 

 vesicle was undergoing retrograde metamorphosis. " Fur 

 die Annahme, dass der Keimfleck, wie das Keimblaschen 

 zerfallt, lasst sich daher keine directe Beobachtung anfiihren." 

 He was equally unable to observe a new formation of the 

 nucleus of the e^g. 



(5.) The nucleus of the egg at the moment when it 

 appears is situated near the germinal vesicle ; the Wagner's 

 spot at the moment of its disappearance is adjacent to the 

 yolk. 



Hertwig has not then observed directly the transformation 

 of Wagner's spot into the body which he calls the nucleus of 

 the eq,^ ; he has never seen Wagner'' s body leave the germinal 

 vesicle in order to penetrate into the yolk. Hence some doubt 

 must remain as to the identity of these two elements, what- 

 ever may be the arguments by which he seeks to establish 

 this conclusion. Hertwig himself acknowledges this fact 

 when he writes : " Bei Abwagung aller dieser Verhaltnisse 



