DOLIOLL'M LARVAL DEVELOPMENT. 



387 



Uljanin) in which we perhaps have the homologue of the ganglionic 

 cell-strand discovered by van Beneden and Julin in tlie Ascidians 

 (p. 376). The peripheral nerves and the sensory organs develop 

 later, and among these the vesicular auditory organ which belongs 

 to the left side of the body deserves special mention (Fig. 245 A, <>t) ; 

 the vesicle itself arises as an ectodermal invagination into which a 

 cell wanders and develops into the otolith. According to Uljanin, 

 the auditory organ of Doliolvmn Mullen remains throughout life a 

 mere cup-shaped ectodermal invagination. 



ms 



ms' 



Ki'.. 182. Transverse section through two ontogenetic stages of Doliolum (after 

 I i j \nixi. . I. section through anterior region of the body at a stage somewhat 

 holder tan that depicted in Fig. 17'- 1 .1 ; 11. section through an older stage, d, 

 paired pharyngeal outgrowth, which takes part in the formation of the ventral 

 stolon; ec, ectoderm; en, entoderm; h, rudiments of the heart and pericardium; 

 ms, mesoderm ; ms', mesoderm of the ventral stolon : n, rudiment of the nervous 

 -\ stem ; p, pharynx. 



The mesoderm of the anterior region of the body gives rise 

 principally to the muscle-hoops (Fig. 181, m), the pericardial rudi- 

 ment (Fig. 182 B, h) and the mesoderm-mass (ms) of the ventral 

 proliferating stolon of the "nurse" stage (the rosette-shaped organ 

 of KEFERSTEIN and Ehlers). Two cell-groups become separated 

 posteriorly and Neutrally from the mesoderm-layer which envelops 

 the pharyngeal cavity like a mantle; one of these, in close contact to 

 the ectoderm, becomes the mesoderm of the ventral stolon (ms'), 

 while the Other, near the rudiment of the alimentary canal, changes 

 into the pericardial vesicle (h), a cavity appearing within it round 

 which the cells become arranged into an epithelium. As in other 

 Tunicates, the heart is derived from a dorsal invagination of this 



