104 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY, 
for some such process is evidenced by the enormous development of 
larval excretory and respiratory organs in these encapsuled embryos ; 
for it is in these forms that the primitive kidneys reach a maximum 
development, and the pulsating head vesicle and «pedal sinus (podocyst 
or amnion of Jourdain) are found. 
A large amount of yolk in forms subjected to a fresh-water environ- 
ment, as Neritina, may conduce to the suppression of this ephemeral 
recurrent cavity. Its action may be purely mechanical, or it may per- 
haps be physiological, in that the process of cleavage, and therefore 
that of metabolism, take place less rapidly in such forms than in cases 
where there is less yolk and approximately equal cleavage prevails. 
In conclusion, then, it may be said that the existence of a cleavage 
cavity is dependent more upon the physiological necessities of the egg 
than upon the internal processes of cell division, or the mechanical neces- 
sities of cleavage, and that its significance is preeminently physiological 
rather than morphological. 
3. Experimental, 
Fortunately the effect of salt in the surrounding medium is a matter 
that can be tested by actual experiment. As the eggs of Limax must bo 
placed for examination in water, —an unnatural environment, — it has 
seemed best not to employ these, inasmuch as the water itself introduces 
a disturbing factor. Water very readily penetrates the membranes of 
the egg of Limax, and the albumen absorbs it so that the eggs become 
quite turgid. Accordingly, recourse was had to the eggs of fresh-water 
forms where it is not necessary to remove the eggs from their natural 
environment for experimentation. For this purpose the eggs of Amnic- 
ola limosa and Physa heterostropha were used. An egg mass of Physa 
showing the early stages of cleavage was divided into two equal parts, 
one of which was kept in the normal water of the aquarium while 
the other was placed in a salt solution made from the same water. 
The eggs were then kept under continuous observation, the phases of 
cleavage, and the size of the cleavage cavity in units of the ocular 
micrometer were recorded for both lots of eggs. There is some variation 
in individual eggs as to the rate of development, so that the observation 
of a number of eggs is desirable, in order that the predominant condition 
may be taken as the typical one. 
The eggs of Physa in the four-cell stage with a maximum cavity were 
placed in 0.75% salt solution, The elimination of the contents of the 
cavity occurred very soon afterwards, and slightly earlier in the eggs in 
