DEVELOPMENT OF THE EUROPEAN OYSTER. 345 



begins gradually to lose its original character of a glandular 

 invagination, flattens out, and passes into a saddle-shaped 

 thickening of the epiblast, consisting of long columnar cells 

 (fig, 9 5^). The secretion of these cells gives origin to a 

 thin cuticular membrane, which forms the rudiment of the 

 shell, because in the full-grown animal the shell-hinge is 

 situated on the same point ; the assertion of Davaine — " Un 



trait transparent c'est le premier indice de la char- 



ni^re" — was quite right. The bivalve shell of Ostrea is, 

 therefore, undoubtedly an unpaii-ed formation, and is not 

 developed, as suggested by Lacaze-Duthiers, of two separated 

 halves, which afterwards unite and form a hinge. According 

 to Brooks^ observations the shell of the American oyster 

 also is formed at first of two separated valves, which develop 

 from a small, irregular, transparent body, situated on both 

 sides of the dorsal pit (His blastopore). 



Considering, however, as I have demonstrated before, that 

 the true signification of that pit and the real blastopore have 

 probably escaped him, we may doubt a little the exactness 

 of his investigations. Moreover, my observations so entirely 

 agree with Hatschek^s on the first apparition of the shell in 

 Teredo that it seems permitted to conclude that in all the 

 Molluscs the shell is formed in the same way. 



With regard to the internal changes of the embryo we 

 observe in fig. 9 that between the two layers of hypoblast 

 and epiblast a space is formed, the body cavity, which con- 

 tains several mesoblastic cells. In this stage also the velum 

 makes its appearance, as a ring of cilia, situated in the 

 prae-oral region, which consists of large cylindrical cells. 



Afterwards the shell is increased rapidly in size, and 

 covers a great part of the body of the larva (fig. 10) ; 

 treated with sulphuric acid it becomes clear that it contains 

 already calcareous salts. The velum developes more 

 and more (fig. 11), and in the midst of the cephalic area, 

 which it surrounds, a thickening of the epiblast appears 

 (Scheitelplatte). The infundibuliform oesophagus leads to 

 a pear-shaped stomach, while the intestine gets an external 

 opening, leading into the pallial cavity. 



At a later period (fig. 12) several parts of the larval body 

 (the cephalic thickening, the oesophagus, the stomach) be- 

 come provided with a reddish and black pigment, which 

 gives the at first colourless larvaj (white spat) a greyish-blue 

 colour (black spat). Beside the alimentary tract in the 

 body-cavity there appear ramified muscular fibres, a ventral 

 and a dorsal one {vs and ds), which originate beside the hinge 

 and attach themselves at the inner side of the velum, which 



