ASCIDIACEA DEVELOPMENT OF THE FREE- SWIMMING LARVA. 363 



somewhat loosened ; they assume a spherical form and constitute a 

 mesenchyne which fills the primary body-cavity of the trunk-section 

 (Fig. 168, m). 



We have seen (p. 350) that, according to VAN BENEDEN and JULIN, 

 the mesoderm in this anterior part becomes detached from the arch- 

 enteron in the form of paired coelomic 

 diverticula (Figs. 159 and 160 A). The 

 true coeloin that arises in this way 

 belongs, however, merely to the earliest 

 embryonic stages, and disappears even 

 as early as the time when the mesoderm 

 completely separates from the entoderrn. 

 The mesoderm then fills the space 

 between the ectoderm and the entoderm, 

 which, in later stages, becomes consider- 

 ably widened and, as it appears, filled 

 with a gelatinous mass ; this forms the 

 ground - substance of the mesenchyme 

 that arises through the transformation 

 of this anterior portion of the mesoderm. 

 The lacunae which arise later in this 

 mesenchyme must be regarded, like 

 the blood-vessels (which, according to 

 VAN BENEDEN and JULIN are entirely 

 devoid of an endothelial wall) as the 

 pseudocode. 



The mesenchyme of the trunk-region, 

 which is derived in the above way, yields 

 the mesodermal organs of the adult 

 Ascidiau. Its histological differentiation 

 gives rise to the connective tissue, as 

 well as to the pigmented elements, 

 and to the body-musculature of the 

 adult which appears arranged in radial 

 and circular muscles surrounding the 



inhalent and exhalent orifices as well as into longitudinal muscles 

 of the trunk, etc. Single cells of the mesenchyme, which become 

 free and reach the pseudocode, become changed into blood- 

 corpuscles. We shall see later (p. 380) that the genital organs 

 also, with their efferent ducts, and the urinary organs originate in 

 the mesoderm. 



FIG. 169. Transverse section 

 through the caudal portion of 

 the free - swimming larva of 

 Clavelina (after SBELIGER). ch, 

 chorda ; ec, ectoderm ; fl, 

 median fin ; ?>*, cellulose 

 mantle ; mf, muscle-tibrillae in 

 transverse section ; mz, muscle- 

 cells ; nr, neural tube ; s, sub- 

 chordal entoderm - strand ; z, 

 mantle cells. 



