246 CLA VYPOLE. [Von. XIV. 
in its history. Usually it is present only for a short time just 
before yolk formation. 
There can be no doubt that the yolk nucleus has an impor- 
tant part to play in the developing egg, and that its function 
concerns the formation of yolk, as agreed by Stuhlmann and 
Schiitz. Balbiani, Sabatier, and Jatta formerly held a view 
regarding it as a fertilizing element, precursor of the sperm 
cell, but this has long been abandoned. A question naturally 
arises as to the origin of this necessary body, for which some 
name ought to be found excluding the term nucleus with which 
it has nothing in common, or very little, even if its nuclear 
origin were demonstrated. Balbiani, working chiefly among 
arachnids, where the yolk nucleus is largely developed, sug- 
gested its homology with the centrosome of the spermatozoan 
and somatic cells. Agreeing with Boveri that the centrosome 
has no part to play in the female cell, he considers the yolk 
nucleus as a case of a true hypertrophy of degeneration. It is 
easily supposable that in the process of degeneration some 
ferment should be originated useful in yolk formation; hence 
its preservation under anew form. Its appearance in the ovum 
just previous to the beginning of growth may indicate the fact 
that the period after the last division has taken place is when 
degeneration of the centrosome sets in, accompanied by its 
useful hypertrophy. Balbiani’s figure of Geophilus shows a 
strikingly radiate structure in the “yolk nucleus.” 
The late persistence of this body in the egg of Peripatus 
cedwardstt is perhaps explicable on these grounds. This species 
of Peripatus is viviparous, and the appearance of intra-uterine 
development in this form reduced the amount of yolk needed. 
This was consequently decreased, but as yet the reaction has 
not included the formative material; this is still formed, perhaps, 
in unreduced quantities, and hence remains not transformed 
into yolk. It has not yet responded to the changed conditions 
which require a smaller amount of yolk. 
It is interesting to note that the yolk nucleus and its origin 
bring up the much-disputed question as to the origin of the 
centrosome. Balbiani’s arguments demanding the origin of 
the yolk nucleus from the nucleus naturally carry back the 
