450 SUB-PHYLUM UROCHORDA OR TUNIC AT A. 



duct runs by the side of the oviduct. In sotne 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 



NP E 



NCH- 



FlG. 242. Young embryo of Ascidian (Clavelina). After 

 Van Beneden and Julin. 



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

 ectoderm ; M. t mesoderm ; A. t archenteron. 



the neuropore, and posteriorly communicates with the archenteron by 

 the neurenteric canal. 



With regard to the origin of mesoblast and notochord, there is more 

 difficulty. Both originate from the endoderm in "the region of the 

 blastopore, and for a time grow forward together. The notochord lies 

 in its usual position on the roof of the gut, from a specialisation of 

 which it arises ; but its forward extension is limited, it never extends 

 into the anterior region, and in the posterior region the future tail it 

 incieases at the expense of the primitive gut, whose lumen it obliterates. 

 The mesoderm, on the other hand, extends right forward, and becomes 

 divided into two regions a posterior, ultimately forming the muscula- 

 ture of the tail, and an anterior, giving rise to the blood, connective 

 tissues, body muscles, excretory and genital organs. According to Van 



