92 bulletin: museum of comparative zoology. 



cells, and the embryo is much flattened in the dorse-ventral direction, that 

 he says a distinct " segmentation cavity, or more properly a body cavity, 

 is now clearly visible " between the ectoderm and the entoderm. Horst 

 ('82) says, " It is not possible to demonstrate the existence of a true 

 cleavage cavity in the oyster." 



Hatschek ('80) finds that one of the peculiarities of the development 

 of Teredo is "der ganzliche Mangel der Furchungshohle." 



II. Fresh - Water Forms. 



Forel ('68, p. 14) called attention to the fact that the "yolk " in Unio 

 later becomes clear and transparent, but he failed to interpret this 

 appearance as a cavity. It remained for Flemming ('75) to show in 

 Anodonta the presence of a cleavage cavity as early as the two-cell stage, 

 to establish its recurrent character, and to assert its equivalency to the 

 cleavage cavity of the later stages. He notes its formation as a lens- 

 shaped cavity between the macromere and micromere of the two-cell 

 stage ; its disappearance with approaching cleavage ; its reappearance 

 in the four-cell stage, and its subsequent disappearance before the next 

 cleavage begins. He also notes its relatively large volume in a stage 

 when thei'e are from six to ten micromeres, but he does not describe 

 any further obliterations of the cavity. The fact that the earlier cavities 

 are obliterated does not raise the question with him as to whether they 

 should be regarded as cleavage cavities or not. 



Eabl ('7G) has observed a similar phenomenon in TJnio pictorum. He 

 describes the cavity as a transparent protoplasmic layer with few yolk 

 granules between the cells of the two-cell stage. He contends, however, 

 that it is not the beginning of the cleavage cavity, as Flemming had 

 maintained, and calls attention to the fact that similar regions, free 

 from granular structures, between two or more cleavage spheres, are 

 met with in the embryos of other animals, as, for example, in many Gas- 

 teropods, where the cleavage cavity appears later than it does in Unio. 

 At the four-cell stage, however, he recognizes " die erste Anlage der 

 Furchungshohle," but does not speak of any reduction or disappearance 

 of this cavity in later stages, though some of his figures suggest it. 



Korschelt ('91) speaks of an expansion of the primitively narrow seg- 

 mentation cavity of Dreissena, during which the embryo, which has 

 reached the gastrula stage, assumes a roundish oval shape. No mention 

 is made of the ephemeral or recurrent character of this primitive cavity. 

 It is a matter of considerable interest to see that Dreissena, which is a 

 " near ally of the common mussel," and is probably a recent migrant 



