DEVELOPMENT, 



431 



animals. It has been suggested that the sub-neural gland 

 may have some renal function. 



Reproductive system. Tunicates are hermaphrodite. 

 The reproductive organs (Fig. 209, G.) are very simple, and 

 lie in the loop of the intestine. The ovary is the larger, 

 and contains a cavity into which the ova are set free, and 

 from which they pass outwards along an oviduct which 

 opens into the cloacal chamber. The testis surrounds the 

 ovary, and is mature at a different time (dichogamy) ; its 



NC 



NP E 



M 



NCR 



E 



FIG. 211. Young embryo of Ascidian (Clavelina). After 

 Van Beneden and J ulin. 



NP., Neuropore ; NC., neural canal ; NCH., notochord ; E. , 

 ectoderm ; AS. , mesoderm ; A., archenteron. 



duct runs by the side of the oviduct. In some forms, where 

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

 ova are surrounded by follicular cells, and probably fertilised 

 in the cloaca. 



Development. The fertilised ovum divides completely and almost 

 equally. The spherical blastosphere becomes slightly flattened, and 

 ultimately forms a two-layered gastrula. 



Along the dorsal median line of the gastrula the ectoderm cells form 

 the medullary groove, the sides of which arch together and form a 

 canal the medullary canal. This opens anteriorly to the exterior by 

 the neuropore, and posteriorly communicates with the archenteron by 

 the neurenteric canal. 



