THE DEVELOPMENT OF THE TEMNOCEPHALE^. 417 
bridge over, but ratlier to widen, the gap between the 
Temnocephaleae and neighbouring groups. 
II. Methods. 
Though the eggs are abundant and readily procurable, the 
study of the development of Temnocephala presents 
considerable technical difficulties, owing to the intractable 
character of the material. The egg-shell is toug'h and rela- 
tively thick, and not readily j^erineable by reagents. When 
it is broken through, the content.s, in the fresh condition, 
burst out, and become completely disorganised. When fixed 
and hardened in the ordinary way the yolk becomes ex- 
tremely hard and brittle. Many methods were experimented 
with before the following coui'se of procedure, which has 
proved sufficiently satisfactory, was finally arrived at. 
The eggs are fixed with sublimate alcohol followed by 
iodized alcohol and 90 per cent, alcohol. After hardening 
they are treated with a solution of hypochlorite of soda. If 
the eggs are transferred dii*ectly from the strong alcohol to 
the hypochlorite solution, the shell of most of them splits 
longitudinally, and, before the desired effect in softening 
and removing the egg shell has been attained, the contents, 
completely exposed, are disintegrated. This effect is avoided 
by making the transference gradually through downwardly 
graded alcohols to the wateiy solution. 
A weak solution of the hypochlorite soon dissolves the 
cement that attaches the eggs to one another, and begins to 
act on the substance of the egg-shell itself. When the action 
is judged to have ])roceeded far enough, the eggs are washed 
in distilled water, and then dehydrated with alcohol. Double 
embedding is essential. From absolute alcohol the eggs are 
transferred to a mixture of equal parts of absolute alcohol 
and anhydrous ether, in which they remain for twenty-four 
hours. They then remain for a like period in | per cent, 
solution of photoxyliu (or celloidin) in equal parts of absolute 
