A Comparative Study of the Structure and Origin of the Yolk Nucleus. 707 
It may now reasonably be said of the yolk nucleus what Platner (72) 
said of the Nebenkern, namely: “Der Nebenkern, dieses von la Valette 
St. George zuerst beobachtete, vielfach bestrittene und noch öfter miß- 
deutete Element ist damit aus der Sonderstellung, welche er bisher ein- 
nahm, herausgetreten, und muß in eine Reihe gestellt werden mit der 
von van Beneden in den Furchungszellen von Ascaris megalocephala 
beschriebenen “spherc attractive”, mit ihren “corpuscules centraux”, 
mit dem BovERischen “Archoplasma” und den Periplasten Vejdovskys. 
Ich bin mit van Beneden der Ansicht, daß sich ähnliche Elemente wohl 
noch in allen Zellen nachweisen lassen werden.” 
IV. General Summary. 
1. If we mean by the term, yolk nucleus, anything in the cytoplasm 
which differs in any respect from the egg cytoplasm in general, there are 
at least four different bodies ineluded in that term: 1. real nuclei; 2. karyo- 
lvmph; 3. metaplasm; 4. the centrosphere or vitelline body. 
2. Eggs may devour other cells. But if the egg is normal, such cells 
soon dissolve and leave no permanent trace that c-ould account for the 
yolk nucleus. This statement does not seem to apply to ascidian test cells. 
3. When real nuclei, giving the staining reaction of chromatin are 
found in the cytoplasm, it is evidence of beginning degeneration of the 
egg — not a normal but a pathological effect. 
I have elsewhere expressed the view that these nuclei are an indica- 
tion of a regressive metamorphosis of the egg, when it is retained in the 
ovary beyond the normal time of its discharge. The nuclei may be due 
to phagocytes, or since there are no distinct cell boundaries, they may 
be due to a fragmentation of the germinal vesicle, which no longer exists 
in those eggs. 
Spermatocytes die and disintegrate without the entrance of phago- 
cytes; and in those cases the trouble seems to be in the nucleus, which 
evidently ceases to produce the karyolvmph. 
4. The karyolymph is produced by the chromatin, causing vacuoles, 
which in the normal cell give rise to the nuclear network from the chromo- 
somes after karyokinesis. 
In eggs, the karyolymph comes out from the nucleus, and may form 
a clear zone around the germinal vesicle. But more commonly, it passes 
into the sphere at one pole of the nucleus. 
5. In either ease when the karyolymph comes in contact with un- 
assimilated, ingested food, such as a cell, or other proteid substances in 
