SECT, i] PHYSICO-CHEMICAL SYSTEM 323 



bacteria to grow on pure proteins, and as the jelly contains no 

 enzymes of an autolytic character no protein breakdown products 

 are formed, and consequently no bacterial growth takes place. This 

 might be considered a protection of the developing embryo from 

 bacterial attack. It is very probable, moreover, that the mucoprotein 

 acts as a source of nourishment for the young tadpoles immediately 

 after hatching, for they invariably attach themselves to it after they 

 emerge from the egg-membrane, and hang on to it by their oral 

 suckers (for histological details consult Nussbaum and Lebrun). On 

 the other hand, development will readily proceed in the absence of 

 the jelly, for as Hluchovski has shown it is disintegrated by exposure 

 to ultra-violet light and may thus be removed without harming the 

 eggs. 



The swelling which takes place in the gelatinous covering when 

 the eggs are shed into the water was studied as long ago as 1824 

 by Prevost & Dumas, who measured the size of the eggs at intervals 

 after they were laid. Their table is as follows : 



They observed that dyes would pass through the jelly as soon as 

 it had swollen, but not before. Similar work by Wintrebert on 

 Discoglossus pinctus gave the following figures : 



As regards the mineralogical and morphological structure of the 

 egg-shells of the lower animals, a good deal is known, and for full 

 detail the reviews of Prenant and of Biedermann should be referred 

 to. The majority of reptile egg-shells have their calcium carbonate 

 in the form of calcite, as Kelly; Schmidtt, and Meigen have shown, 

 but the two first-named investigators discovered that the tgg- 

 sheUs of chelonia were of aragonite, and later Lacroix observed 

 a similar phenomenon in the case of certain saurians. The tgg- 



