450 SUB-PHYLUM UROCHORDA OR TUNICATA. 
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 
Fic. 242.—Young embryo of Ascidian (C/aveléna).—After 
Van Beneden and Julin, 
NP., Neuropore; NC., neural canal; WCH., notochord; Z., 
ectoderm ; J7., mesoderm; A., 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 limilted,—it never extends 
into the anterior region, and in the posterior region—the future tail—it 
increases 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 
