418 KING. [Vot. XIX. 
VIII. THE FORMATION OF YOLK. 
One of the most difficult problems met with in the study of 
amphibian odgenesis is that concerning the origin of the yolk. 
This problem includes not only a consideration of the origin 
and nature of the so-called “yolk-nuclei,” but it also involves 
practically the whole theory of cell action since it cannot be 
supposed that the yolk formation takes place independently of 
nuclear activity. The anabolic processes taking place in the 
cell as a result of the interaction of the nucleus and the cyto- 
plasm are as yet very imperfectly understood, and until we 
have obtained a clearer insight into the nuclear-cytoplasmic 
relations it will not be possible to solve the problem of yolk 
formation in an entirely satisfactory manner. 
There is as great a diversity of opinion regarding the nature 
of the yolk-nucleus and the origin of the yolk in the amphibian 
egg as there is concerning the origin of the egg itself. The 
first observation regarding the presence of a yolk-nucleus in 
the amphibian egg were made by Cramer (23) in 1848. Ac- 
cording to this investigator the cell body of the frog’s egg 
contains a small granular ball which later spreads out in the 
form of a half-moon around the nucleus and gives off gran- 
ules which develop into yolk spherules. In 1850 Carus (19) 
investigated the young egg of Rana temporaria and failed to 
find the granular ball described by Cramer. He states that the 
yolk first appears at the periphery of the egg in the form of 
single granules as it does in the egg of Alytes obstetricans 
according to the earlier observations of Vogt (90). 
A few years later Thompson (89) wrote in regard to the 
presence of a yolk-nucleus in the frog’s egg: “I have in gen- 
eral found it present, and think it more probable that it may 
be destined to form the external and larger corpuscles of the 
yolk, while the clearer part immediately surrounding the germ- 
inal vesicle may contribute to the production both of these and 
of the finer substance in which the germinal vesicle is found 
imbedded.” 
As Goette failed to find a yolk-nucleus either in the egg 
of Bombinator or of Bufo, Hertwig (40) is inclined to attach 
