1901.] NATURAL SCIENCES OF PIlILADELl'HIA. 701 



that gradually separate off and become free. At first pear-shaped, 

 with a slender stalk, they later become rounded and are frequently 

 amoeboid in outline, fig. 51, Bl.f. C. The cytoplasm is finely granu- 

 lar in appearance and quite f)rominent. In the anterior (rhyn- 

 choca?lomic) part of the dorsal vessel the blood-forming cells arise 

 only from the dorsal side, but posteriorly from all sides of the 

 dorsal vessel (cf. figs. 50, 51). 



The dorsal blood vessel after leaving the rhynchoccel lies beneath 

 it and above the intestine, figs. 22, 2S, immediately surrounded by 

 a network of connective tissue cells. The wall, fig. 51, consists of 

 an inner endothelium, End., and an outer circular muscle layer, 

 BLM., that in this region is continued around the entire vessel, 

 and is not confined to the dorso-lateral surface, as in the anterior 

 region. The muscle fibres are very fine, but are clear and distinct, 

 and are especially well seen in tangential sections of the vessel. 

 The endothelium. End., is very irregular, being interrupted by the 

 proliferation of cells from all sides of the vessel. In places, many 

 consecutive sections of the vessel may be examined without finding 

 a single true endothelial cell, while numerous blood-forming cells 

 are present in each section. Two explanations for this are possible: 

 either all the endothelial cells have been changed into blood- 

 forming cells, or the endothelium has been rubbed or torn off. The 

 latter is supported by the fact that a thin lining of cytoplasm may 

 nearly always be seen on the inner side of the muscular layer, even 

 though no nuclei are present. 



The two dorso-lateral vessels at the posterior (:nd of the body, 

 figs. 26, 49, have the same histological structure as the dorsal ves- 

 sel — the outer, circular muscle layer, and the inner, more or less 

 interrupted endothelium and numerous blood-forming cells. 



The lateral vessels, figs. 46, 47, L.V., and their ventral con- 

 nectives, V. hi. con., are lined anterioi'ly by low, brick-shaped cells 

 like those of the median vessel, with prominent nuclei and hyaline 

 cytoplasm. This endothelium soon becomes more flattened and 

 membraneous. Here and there a blood-forming cell is given off, 

 fig. 46, Bl.f.C, but there is no abundant proliferation of these 

 cells as in the dorsal vessel. In the nephridial region the endo- 

 thelium of the lateral blood vessels is discontinuous, being absent 

 from the surfaces of the terminal bulbs. Xo muscular layer is 

 present in the anterior part of the lateral vessels, the wall consist- 



