The Prophase in the Ovigenesis and the Spermatogenesis of Plauaria etc. 433 
hinter ihr liegen, findet man auch im Innern der Schnitte au gut 
fixierten Präparaten keine Schrumpfung. ‘‘ 
The artificial character of synapsis is quite evident to Mc Clung 
(’OO) because: „When observed, the mass of the chromatin is always 
to be found in the region of the nucleus opposite to the point at 
which the fixing or dehydrating fluids had free entrance.” 
Tliis has not been my experience, and I shall now give some 
reasons for considering the synapsis a natural condition of the cells 
at that stage in their history in which it occurs. I liave observed 
the synaptic stages in testis cells of the living animal ( PL lactea), 
teased up in normal saline solution containiug a trace of Neutral Red. 
Mr. Walker of tliis Laboratory has also seen the synapsis in the 
living cells of the testes of mouse, guinea-pig, and rat. Immediately 
after the animals had been killed, he took portions of the testes and 
immersed them in normal salt solution, witli a very little Methylene 
Blue. 
The Plauaria are eminently suitable for good and complete 
fixation. 
I have fixed them with Flemming’s strong and weak, IIek- 
mann’s, Zenker’s (my modification) Lo Bianco’s, and concentrated 
Sublimate Solutions. The best of fliese is Flemmixg’s strong solution. 
The animal is put into a watchglass, and all the water drained off. 
Then 3 or 4 ccs of Flemming is quickly poured over it. This causes 
the animal to curl up at first, but it again quickly extends to almost 
its natural lengtb. All this takes place in less tlian 30 seconds, by 
which time the outer skin of the planarian is sufficiently hardened 
to permit of it being raised across a brush, and then cut into small 
pieces from less than 1 mm to several mm thick. In PL lactea , in 
adult specimens, such pieces then are about 3 or 4 mm X 2 min X 5 or 
1 mm. These small pieces are cut while the animal, the scissors, and 
brush, are immersed in the fixative. Can it be said that such small 
portions of the animal bathed on all sides by the fixative are insuf- 
ficiently and badly fixed? It is most unlikely, and even in the very 
t) I have used the following modification of Zenker’s fluid in which Copper 
sulphate is substituted for Sodium sulphate, which seems to counteract the swel- 
ling action of the acetic acid niore completely. 
Potassium Bichromate 2.5 gm 
Copper sulphate 1 „ 
Glacial acetic acid 10 ccs 
in 100 ccs saturated aqueous solution of 
Perchloride of Mercury. 
29 * 
