136 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



well as those of the heart, are derived from the mesoblastic or mesen- 

 chymal structures of the auimal. The radiating muscular bundles of 

 the adult lie just beneath the ectoblast, or epithelium, on the outer sides 

 of the mantle leaves. 



The muscular fibers of the walls of the heart are not striated, and 

 decussate in every direction. The cavity of the heart is crossed in 

 various directions by muscular bands, and a more or less complete mus- 

 cular septum divides the ventricle in the median line. The heart is 

 therefore approximately four-chambered. 



The mesenchymal or mesoblastic tissues compose the great bulk of 

 the body of the animal, and extend out into and lorm the greatest pro- 

 portion of the thickness of the mantle, and also down into the branchiae, 

 where it forms thick transverse vertical septa between the outer com- 

 poundly ribbed walls of the branchial pouches lined with ectoblast. 

 The mesenchyme also gives support to all the viscera, the stomach, 

 liver, intestines, and reproductive organs being embedded in it. 



The branchial blood-channels are also limited by the mesenchyme. 



The mesenchymal cells are large, and will average sthjth of an inch 

 in diameter. They inclose, in all cases, both in winter and summer, a 

 large irregular nucleus, from which a complex network of intracellular 

 fibrils radiate in all directions through the enveloping cellular substance. 

 At one side of the nucleus there are always one or more accessory bod- 

 ies perfectly globular. The mesenchymal elements are not fat cells, as 

 has been erroneously supposed by Brooks. 



The mesenchymal cells are probably very hygroscopic, which explains 

 why it is that oysters may be much swollen by osmose in a short time 

 by immersion in water of less specific gravity than the sea-water in 

 which they grew. 



The. mesenchyme may be regarded as the connective tissue of the 

 animal. It corresponds morphologically to that structure in other types. 



There is an apparent atrophy of the mesenchyme in the body-mass 

 and mantle during the spawning season, with a great concomitant de 

 velopment of the reproductive follicles or tubules. In winter the repro- 

 ductive follicles atrophy, when the mesenchyme again increases in bulk 

 in the body-mass and mantle. These facts appear to show that the 

 reproductive elements are derived from the mesenchyme by a transfor- 

 mation of its substance in which their follicles are embedded. 



It is the great development of the mesenchymal substance in the au- 

 tumn and winter, when the reproductive function is in abeyance, that 

 constitutes the condition of the animal known to oysiermen as " fatness." 

 This word expresses the condition well enough, practically, but it is 

 scientifically incorrect, since there is scarcely any fatty substance in the 

 animal at any lime. 



In summer, when the reproductive organs are gorged with their pro. 

 ducts, their follicles are crowded together into contact ; in winter, in 

 their atrophied condition,. they lie embedded in the superficial portion 



