492 



TUNICATA. 



(Fig. 253, fg). The posterior blind end of this tube gives rise to the 

 subneural gland. 



Isolated mesenchyme-cells are early met with in the primary body- 

 cavity of the stolon (Fig. 257). Seeliger traces these to a great 

 extent to cells of the genital strand that have become independent 

 (Fig. 255 B). Some of these mesenchyme-cells become transformed 

 later into connective tissue-cells and blood-corpuscles. Others take 

 part in the formation of the lenticular and elongate cell-masses. The 

 elongate mass which belongs to the dorsal region (Fig. 253, dm) seems 

 to be connected with the formation of blood-cells. The cells of the 

 lenticular masses (Fig. 253, Im), on the contrary, contain oil globules 

 in their protoplasm in later stages, and are regarded as phosphorescent 

 organs. The elaeoblast also (Figs. 253, eb, 25S, 259) arises from an 



Fig. 259.— Stolon of Pyrosoma with the rudiments of two individuals, / and // (after 

 Seeliger). <■/, atrium; eb, elaeoblast; ed, intestine; es. endostyle; g, genital 

 strand; i, branchial aperture; ks, gill-clefts; m, stomach; ■/. neural tube of bud 

 /, /'. rudiment of ganglion of bud // , oe, oesophagus; s, lateral nerves. 



originally paired accumulation of mesenchyme-cells lying immediately 

 below the entoderm in the distal region of the stolon-segment, The 

 two halves of the elaeoblast-rudiment unite later at their upper 

 and lower surfaces, a perfect circle being thus formed which soon, 

 through deposits of nutritive material, develops the typical character 

 of elaeoblast-tissue. 



The musculature also which, in Pyrosoma, is reduced to a few slight 

 strands running round the branchial and atrial apertures and in the 

 outer wall of the peribranchial cavity, is derived from the mesenchyme- 

 eells. These at first unite to form cell-strands in the peripheral parts 

 of which the fibrils of contractile substance first appear. As these 



