284 JORDAN. [Vol. VIII. 



The stage of pre-eminently cytoplasmic phenomena to which 

 we first turn is distinguished in amphibian ova by the appear- 

 ance and elaboration of the yolk, and by the advent of certain 

 enigmatical structures, the so-called yolk-nuclei. 



Yolk-nucle2is. — The so-called yolk-nucleus (Dotterkern) of 

 the amphibian ^^g appears to have been first observed by 

 Cramer ('48) who discovered it in the ovarian eggs of Rana 

 teviporaria in 1846. He describes it as a "little ball of 

 granules," which, as the Q,g^ becomes larger, spreads out into 

 "an elegant half-moon" around the germinal vesicle, and, 

 finally degenerating, mixes completely with the substance of 

 the ^g%. In the previous year a similar body had been seen 

 by V. Wittich ('45) in the ovarian eggs of spiders, but whether 

 this concentrically ringed structure is in any way comparable 

 with the body found in the amphibian ^^^ was then and is still 

 to this day an open question. Further studies by v. Wittich, 

 V. Siebold and Cams failed to shed much light on the signifi- 

 cance of this concentric body in spiders' eggs. v. Wittich 

 ('49) found it upon the surface of the yolk in freshly laid spider 

 eggs, and speaks of it as " a hollow, thick-walled capsule filled 

 with liquid." Carus ('50) was the first to apply the name 

 Dotterkei'ti to this element of the o^gg. He refers to Cramer's 

 observations upon the similar body in the ^^g of the frog, and 

 is inclined to homologize the two structures. 



Practically nothing was added to these early and frag- 

 mentary observations upon the yolk-nucleus until the investi- 

 gations of Lubbock and of Gegenbaur in 1861. Gegenbaur 

 ('61) discovered in the avian ^g^ " einen fast scharf umschrie- 

 benen runden Fleck . . . mit dem Keimblaschen hat er keine 

 Beziehung, denn dieses liegt immer entfernt von ihm." Ge- 

 genbaur does not commit himself to any definite view on the 

 significance of this " constant element " of the egg, but is 

 evidently inclined to regard it as in some way connected 

 with the origin of the yolk granules. 



Lubbock ('6I) described and figured a body which he ob- 

 served in young Myriopod eggs of several genera, but was 

 doubtful as to its homology with the concentric structure 

 observed in Arachnid eggs, and prudently refrained from at- 



