5o6 



UROCHORDA 



where the gonads are near the cloaca, there are no ducts. 

 The ova are surrounded by foUicular cells, and probably 

 fertilised in the cloaca. 



The development shows three steps : — (i) from the fertlHsed ovum 

 to the free -swimming larval stage ; (2) the larval or so-called " tad- 

 pole " stage ; (3) iixation and degeneration into the adult Ascidian. 



The spherical blast ula forms a gastrula by invagination. Over- 

 growth of the gastrular lip results in the covering over of the dorsal 

 nerve rudiment and the formation of an ectodermic neiural canal, 

 open in front at the neuropore and for a time communicating behind 

 with the archenteron. The embryo elongates. Internally the endo- 



NP E 



NCH 



Fig. 287. — Young embryo of Ascidian [Clavelina). — After 

 Van Beneden and Julin. 



NP., Neuropore ; N.C., neural canal ; NCH., notochord ; E., 

 ectoderm; M., mesoderm ; Jl., archenteron. 



derm of the archenteron gives rise to the notochord. Mesoderm cells 

 spread between the archenteron and outer ectoderm and form the 

 side muscles of the growing tail. The neuropore closes and the neural 

 canal becomes a spinal cord, ending in front in a closed brain vesicle 

 where a dorsal unpaired eye and balancing organ develop (Fig. 288). 



In the free-swimming larva a mouth opens into the pharynx in 

 front of the brain vesicle. On either side an ectodermal invagination 

 appears — these later coalesce dorsally and form the peribranchial 

 chamber of the adult — and fuses with outgrowths from the pharynx 

 where the first two pairs of gill-shts break through. The tadpole- 

 like larva, with dorsal tubular nervous system, notochord in the tail 



