BULLETIN OF THE UNITED STATES FISH COMMISSION. 137 



of the mesenchyme of the body -mass the same as in summer, but are 

 much less developed, so as to appear in .sections like a very open net- 

 work of strands of very small nucleated inc pient embryo cells, the 

 connection of which may be traced into the now collapsed and inter- 

 nally ciliated branches of the oviducts. All the parts of the reproduc- 

 tive apparatus are therefore present in winter, but in an undeveloped 

 condition. 



The oviducts branch 'and spread over each side of the body-mass just 

 extern, d to the stratum of reproductive follicles and immediately beneath 

 the mantle. They do not ramify through the substance of the repro- 

 ductive organ, but lie externally to it. Their principal openings on each 

 side of the body-mass pour their contents into the suprabrancnial 

 chambers on each side just below the muscle. 



The liver is a diverticulum from the entoblastic walls of the stomach. 

 The great bile ducts pass outward from the cavity of the stomach, and 

 subdivide again and again, and end blindly in spacious ovoidal hepatic 

 follicles, the simple walls of which consist of hepatic cells. The func- 

 tion of the liver is in all probability both excretory and secretory, and 

 takes an all-important share in the processes of digestion. 



The entoblastic wall of the intestine is folded inwards at oue side for 

 its entire length in a peculiar way, so that its lumen is more or less cres- 

 centic in cross section. 



There are neither annular nor longitudinal muscular fibers in the wall 

 of the intestine ; the sole motive force in the propulsion of the ingested 

 food appears to be exerted by the ciliary covering which clothes the in- 

 ternal surface of the alimentary tract from Ihe mouth to the anus. 



The words ento- and ectoblastic as applied to the adult, correspond to 

 the embryonic epi- and hypoblast; mesenchyme to the mesoblast. I 

 have adopted the terms from a paper on the mesoblastic layers of em- 

 bryos by R. and O. liertwig, in the Jenaische Zeitschrift (XV, 1st lift., 

 1881), in which they discuss for the first time what they designate as 

 the Ccelom tlieory, as applied to an explanation of the origin and typical 

 forms of mesoblast as derived from the gastrula stage, typical of all 

 metazoau development. The body of facts which is brought forward 

 embraces the results of the work of the principal embryological author- 

 ities, and, although they had already been interpreted in a somewhat 

 similar way by Huxley and McAlister, are for the first time connectedly 

 stated so as to be of fixed value in embryological studies. They have 

 given us a topography of the embryonic layers of the greatest value, 

 which enables us to decipher with the greatest ease the relations and 

 genesis of the parts of a form as comparatively undifferentiated as the 

 adult oyster. In that they have considered Chiton as a member of their 

 second subdivision of the Metazoa, the Pseudoccslia, characterized by the 

 genesis of the mesoblast by proliferation from the epiblast and hypo- 

 blast, a massive, soft, ami unsegmented, bilaterally symmetrical body, it 

 is clear from what has preceded that the oyster is also typical, and a 

 member of this group. 



