SWAMMERDAM 197 



he says, of fibres secreted by the mucous glands of 

 the skin, especially by the thickened edge of the mantle ; 

 these fibres coalesce to form membranes or laminae, 

 which are afterwards calcified. Bones, teeth and the 

 shells of crustaceans are, he thinks, formed in much 

 the same way, and injuries to the shell are repaired like 

 fractured bones. He thinks it wonderful that the shells 

 of aquatic snails and the cocoons of some aquatic insects 

 should form under water, for he attributed the hardening 

 of the mucous secretion to the action of the "ambient air." 

 A number of other mollusks, chiefly land or freshwater 

 species, are described with more or less detail. The 

 account of Paludina is interesting.^ Swammerdam 

 notes the horny operculum, the eyes on lateral pedicels 

 outside the tentacles, and the lingual ribbon. He grows 

 enthusiastic about the viviparous reproduction of this 

 snail, of which he gives a circumstantial account. 



The Frog and the Tadpole 



This memoir ^ is a marvel of patience and anatomical 

 skill. The passage of eggs from the ovary to the exterior 

 of the body is discussed by Swammerdam at great length, 

 but he could not make out how they gain the narrow 

 mouth of the oviduct (a question which still has its 

 difficulties). A large and elaborate drawing displays 

 the anatomy of a tadpole furnished with internal gills. 

 The early stages of development are described, the 

 process of segmentation naturally escaping observation, 

 and a stage is expressly mentioned in which the embryo 

 consists entirely of small granules [greynkens or Moot- 

 hens, our cells).^ The " vermiculi viventes " which 



^Bihlia Naturce, p. 169. ' Biblia Naturae, pp. 789-860. 



^Leeuwenhoek made such a stage known in 1688, eight years after Swam- 

 merdam's death, but long before the publication of the Biblia Natures {infra, 

 p. 203). 



