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SUMMARY OF CURRENT RESEARCHES RELATING TO 
Development of Cerata in iEoiss.* — Mr. C. B. Davenport gives a 
short account of his observations on this phenomenon. He finds evi 
dence that the cerata arise from mesenchyme in two sets of facts ; 
(l) the first indication of the formation of a new ceras is seen in the 
thickening of the mesenchyme at the base of the next older ceras. It is 
not until a solid mass of mesenchymatous tissue has been produced that 
the ectoderm begins to evaginate ; and it then looks as if it were being 
pressed outwards. The alimentary diverticulum is produced still later. 
In the second place it is shown that the ceecum does not take the 
initiative by the discovery of young cerata composed only of ectoderm 
and a thickened mesenchymatous core, without any penetration of endo- 
derm. Embryonic mesenchyme may be found lying at the base of the 
dorsal papilla. 
The author’s observations seem to show that the embryonic or growth 
tissue of JEolis is in its origin identical with that producing sexual cells. 
Like the latter it is germ-tissue, and it differs chiefly from the sexual 
cells in that it gives rise to growths which constitute part of the body of 
the present individual, growths which are as mortal as any other part of 
that present individual. Secondly it differs from sexual cells in giving 
rise to one kind of organ only — the mesenchyme to the mesenchyme of 
the buds, the endodermal diverticulum to the endoderm of the buds. 
In one striking point there is a resemblance between mesenchy- 
matous growth tissue and the sexual cells ; the former, though it goes to 
produce the mesenchyme of any ceras, is not used up in doing so, part 
remains behind to form a new ceras, the rudiments of other new cerata. 
But to this power there is a limitation, which is a matter of degree in 
different species. In any case the limitation in the reproduction of 
cerata must be considered as resulting not from the limited capacity 
of reproduction of the embryonic tissue but from the needs of the 
species. 
5. Lamellibranchiata. 
Morphology of Lamellibranchiata.t — Dr. J. L. Kellogg has made 
use of the section method, which has as yet been but little applied to 
the Lamellibranchiata. Of the conclusions to which his observations 
have led him we may note the following. 
There seems to be a correlation between the aborted or absent foot 
thick mantle with no large blood-spaces, as, also, between a fully 
developed locomotor foot and a mantle consisting mainly of immense 
blood-spaces. Very large man tie- chambers seem to be characteristic of 
those forms which are most active, as Yoldia, Venus , or Pecten. Foot- 
less forms, such as the Oyster, appear to be degenerate, and to have 
come from active ancestors with a fully developed locomotor foot. In 
an active form, such as Venus, a great deal of oxidation goes on in the 
tissues, and this would require corresponding facilities for the aeration 
of the blood. When the power of locomotion is lost and the animal is, 
like Mytilus, fastened by a byssus, oxidation is lessened, and the respira- 
tory apparatus is correspondingly reduced. This is well seen in the 
Oyster. 
The gill is so subject to variation by secondary modifications that it 
* Bull. Mus. Comp. Zool., xxiv. (1893) pp. 141-8 (2 pis.), 
f Bull. U.S. Fish Comm., x. (1892) pp. 389-436 (16 pis.). 
