No. 2.] THE DEVELOPMENT OF THE NEWT. 



28'- 



tributing any special importance to it. "I have," he writes, 

 " several times been inclined to look upon it as a mere acci- 

 dental agglomeration of yelk ; but it is, I think, too regular 

 and too constantly present." 



Since 1861 the yolk nucleus has been described and figured 

 by nearly every worker upon Arachnid, Myriopod or Am- 

 phibian ova. An analogous body has also been observed in 

 Crustacea (Reichenbach, '77), in Mollusca (v. Ihering, '77), 

 and many other groups, but it is doubtful if there is any 

 community of relation whatever between these structurally 

 different bodies. With few exceptions these numerous ob- 

 servations have done little towards explaining the significance 

 of the "yolk nucleus" in any one of the groups of animals 

 in which it appears. 



Balbiani ('64^ '64^) was the first to observe the yolk-nucleus 

 in the egg of Myriopods {Gcophihis), where it appeared to 

 him as " une petite vesicule," much smaller than the germinal 

 vesicle, and lying close to the periphery of the egg. He 

 describes circumstantially the formation of a layer of granules 

 around the vesicule and the subsequent migration of these 

 granules to distant parts of the egg. It was chiefly this 

 phenomenon that led him to believe that the small vesicle 

 was the formative center for the nutritive elements of the 

 Qgg. He also advanced the view that the substance of these 

 eo-o-s consists of a "partie germinative fondamentale et d'une 

 partie nutritive, chacune de ces parties se constitue isolement 

 et pour son propre compte." This separation between the 

 germinative and nutritive elements is, according to Balbiani, 

 primordial, and dates from the first stages of the egg. 



In a paper published some time afterwards, Balbiani ('73) 

 modifies his views regarding the "cellule embryogene " as 

 he now prefers to call it. From his observations upon the 

 Teleostean egg he concluded that the cellule has its birth 

 in the epithelium of the follicle — in some such way as 

 van Beneden and Julin ('87), and Morgan ('90) claim is 

 followed in the formation of those strangely similar bodies, 

 the Ascidian test-cells. In some Teleosts Balbiani finds a 

 straight canal or pathway leading from the follicle into the 



