REPRODUCTIVE SYSTEM. 407 



activity may make diffusion easier. It is interesting to find 

 such a plant-like method of storing up, instead of eliminat- 

 ing, waste products in these very passive animals. It has 

 been suggested that the sub-neural gland may have some 

 renal function. 



Reproductive System. 



Tunicates are hermaphrodite. The reproductive organs 

 (Fig. 128, 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 sur- 



p j rounds the ovary, and is 



mature at a different time 



' A """4lBP " (dichogamy); its duct runs by 



fjEME^ the side of the oviduct. In 



some forms, where the gonads 

 are near the cloaca, there are 

 ^_^ ^x \ | no ducts. The ova are sur- 

 ^af"~\ rounded by follicular cells, 



and are probably fertilised in 

 the cloaca. 



A F .- I3 /~ Y( T g x Emb !T; > f Development. Most of the As- 

 Ascidian (Clavelhna). (After ddians exhibit the development with 

 VAN BENEDEN and JULIN.) metamorphosis which is about to be 



/., Neuropore ; nr., neural canal ; described ; a few in which the larvae 

 ch. t notx)chord; ec., ectoderm; en., are retained for a long time within 

 endoderm. t h e k oc jy o f t ^ e mo ther, show a much 



abbreviated life history. 



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. In the posterior region of the gut, at the sides 

 of the blastopore, a pair of diverticula, according to Van Beneden and 

 Julin, grow out. These form the mesoderm ; the endoderm cells 

 between them, roofing the gut, form the rudiment of the notochord. 

 The mesoderm masses and the notochord grow forward together for a 

 time, but later the mesoderm advances much further into the anterior 

 region, the notochord being limited to the tail. The diverticula 

 originally contain each a small cavity the true coelome, but this is soon 



