PYROSOMA DEVELOPMENT OF THE CYATHOZOOID. 



397 



epithelial continuity, so that finally, only a mass of irregularly 

 arranged cells remains to take part in the formation of the mesenchyrne 

 which is developing in the primary body-cavity. A similar disin- 

 tegration is undergone by the median strand which was regarded as 

 the equivalent of the chorda, and which, after the disappearance of 

 its lumen, retains its independence only for a short time, and is called 

 by SALENSKY the axial mesoderm-strand . The right coelomic sac 

 gives rise to the pericardial rudiment (Figs. 192 C, 189, 190, pc\ 



FIG. 193. Two Cyathozooids with their first-formed buds (after KOWALEVSKY, some- 

 what altered). J, with straight stolon; R, with curved stolon; the Cyathozooid 

 is commencing to rise from the surface of the food-yolk (d). d, atrial aperture ; d, 

 t<tod-y<>lk ; en, rudiment of the endostyle ; I, body-cavity of the Cyathozooid; p 

 peribranchia] tul.es; /,<, pericardia! sac of the Cyathozooid; z, cell-zone. 



which soon becomes club-shaped, a dilated, anterior, sac-like part 

 changing into the pericardial vesicle of the Cyathozooid, while the 

 tube that runs backward from this part does not develop further but 

 soon loses its lumen ; the connection of its cells then becomes loosened. 

 It appears that these elements then mingle with the mesenchyme 

 and assist in the formation of the mesoderm of the Ascidiozooid. In 

 the pericardial sac of the Cyathozooid the surface adjacent to the 

 intestinal wall is seen to thicken, this part then becoming invaginated 

 and forming the rudiment of the heart proper. 



