700 PROCEEDINGS OF THE ACADEMY OF [DeC. 



inner surface of which are irregular groups of large raesenchym 

 cells, figs. 27, 28, mes., many of which become detached and float 

 freely in the lacuna. 



Hidolocjii. — The wall of the median blood vessel is a one-celled 

 layer, fig. 29, of low, somewhat flattened cells that are wider than 

 high. The nuclei are oval and vather prominent and the cyto- 

 plasm clear and hyaline. In places the wall becomes a flattened 

 endothelium, often appearing in cross section like a thin membrane, 

 along wh'ch the nuclei lie. 



The lining of the cerebral organ blood vessels is of a similar 

 character. 



The dorsal blood vessel, as already mentioned, lies, in the ante- 

 rior part of its course, in the mid-ventral wall of the rhynchocoel, 

 and posteriorly, beneath the rhynchoccel. 



Fig. 50, a section through the dorsal blood vessel shortly behind 

 its origin, shows that the vessel is surrounded ventrally and later- 

 ally by bundles of longitudinal fibres, which are more numerous 

 here than on the other surfaces of the rhynchoco?l wall, and proba- 

 bly take part in the contractions of the vessel, while the dorsal 

 surface of the vessel is bounded by the epithelial lining of the rhyn- 

 chnccel, Rc.Ep. 



The wall of the dorsal vessel consists of an endothelium. End., 

 of low, rather brick-shaped cells with large nuclei. The cells of 

 the ventral part of the wall are very regularly arranged, Plate 

 XLIIl, fig. 50, but the regularity of the dorsal wall is interrupted 

 by the proliferation of numerous blood-forming cells, Bl.f. C, that 

 project into the lumen of the vessel and remain for some time at- 

 tached to the dorsal wall by their slender stalks. 



The boundary between the dorsal wall of the blood vessel and 

 the epithelium of the rhynchocud is marked by a fine line, Bl.M., 

 that varies in distinctness in different preparations. After the 

 luematoxylin-eosin stain it appears pink, after iron-hrematoxyliu, 

 black, so that it is probable that a few muscle fibres are present 

 here, forming a very delicate circular layer around the dorsal side 

 of the vessel. This line might also represent a deeply stained 

 connective tissue layer, but, since the dorsal vessel after leaving the 

 rhynchocfel has a well-defined muscle slieatli, it seems more likely 

 that this is the beginning of a muscle layer. 



The blood-forming cells are evidentlv enlarged endothelial cells. 



