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 be 

 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 



