DOLIOLUM LARVAL DEVELOPMENT. 



385 



by means of its long caudal region, and is therefore usually called 

 a him i although it is still enveloped in the much-distended egg-shell 

 (t/t) in which traces of the follicle-cells can be found. We do not 

 know for certain when this egg-shell is cast off. During these pelagic 

 ontogenetic stages in which the Doliolum resembles the Ascidian 

 larva, the body is elongate (Fig. 179 A) and the middle of it is 

 occupied by a vesicular ectodermal swelling (eb) caused by the 

 accumulation of a clear fluid. This vesicle divides the body into a 

 posterior and an anterior region. The anterior develops into the 

 young Dnl lot-urn (the first asexual or "nurse" form, the blaatoeooid) 

 while the ccrodermal vesicle and the caudal region must be regarded 



TL 



KK;. 180. Young " mirst; " t'onn <>f l)li<,/n m l-'Ji renberyii, with remains of the larval 

 tail (after ULJANIN). ch, chorda ; rf, so-called dorsal stolon ; e, endostyle ; h, heart 

 and pericardium ; ///, egg-shell ; /?, nervous system ; r, rosette-shaped organ (rudi- 

 ment of the ventral stolon). 



as provisional larval organs and degenerate later (Figs. 179 B, and 

 180). The structure of the caudal region corresponds to that of 

 the same region in the Ascidian larva. It consists of a chorda and, 

 laterally, of muscle-plates derived from the mesoderm-bands. . At the 

 anterior end of the caudal region, a part of the mass of mesoderm- 

 cells (Fig. 1 79, mx") is not transformed into spindle-shaped muscle- 

 fibres. Two cell-masses (y) are subsequently given off from this into 

 the ectodermal vesicle, where they break up and change into blood- 

 corpuscles. 



The anterior region of the body contains the very large rudiment 

 of rhe central nervous system (Fig. 179 A, n) and the anterior 



cc 



