BULLETIN OF THE EXITED STATES FISH COMMISSION. 135 



rcss Monroe for supplies. It seems that the stewards trusted somewhat 

 to the resources of the places we expected to visit when laying iu stores, 

 and now find themselves running short. 



We will go either to York River or Cherrystone to-night, being gov- 

 erned by the weather. We can do nothing with nets at the latter place, 

 unless it is calm and smooth, as they are to.be set in the open bay. 



Should we rind anything iu York River it will be an inducement to try 

 our fortune in Mob Jack Bay: if not, it will hardly be worth while. 



Our present plan is to spend about three or four more working-days 

 in the bay and Lower Potomac, and then return to Washington. 



1.45 p. m. — Arrived at Fortress Monroe. Will leave about 2.30 for 

 Cherrystone. 



Steamer Fish Hawk, Tangier Sound, March 2, 1882. 



PRELIMINARY NOTICE OF SOME POINTS IN THE MINUTE ANA- 

 TOMY OF THE OYSTER. 



By JOHN A. RYDER. 



There is a spacious segmentation cavity developed iu the embryo 

 which becomes the subdivided body cavity (schizoccel) of later stages. 



Between the ecto- and entoblast the mesenchymal or mesoblastic tis- 

 sue is developed, apparently, and for comparative embryological reasons, 

 from the two former by proliferation, whereby the segmentation cavity 

 becomes in part obliterated. 



The mesoblast of the embryo, formed as above stated, is the tissue 

 from which the mesenchyme or connective tissue of the adult is devel- 

 oped. 



The blood channels, or canals, are developed in the mesenchyme; no 

 specialized endothelial lining cells are ever differentiated, the mesen- 

 chymal cells form their immediate walls. An exception to this is found 

 only in the structure of the anterior and posterior aorta?, the heart, and 

 branchio- cardiac vessels, which have proper walls. 



Iu some places the mesenchymal tissue is found to be spongy, its cells 

 being built around complex anastomosing spaces for the blood. There 

 is, therefore, a true schizoccel developed in the oyster; -it has been formed 

 as the mesenchyme has grown into the segmentation cavity, which has 

 been subdivided in this way into haemal canals and spaces as develop- 

 ment proceeded. 



The heart, as in the embryo fish, develops in the blastoccel or segmen- 

 tation cavity, the pericardiac space being a remnant of the latter, (dee 

 Davaine, Recherches sur la Generation des Huitres. Mem. de la Soc. 

 de Biologie IV, Paris, 1853. The foregoing statement is made upon the 

 evidences afforded by the observations of the author above cited, ou the 

 development of the heart.) 



The adductor of the shell and the radiating muscles of the mantle, as 



