. Outlines of zoology. stem.—Tunicates are reproductive organs (Fig. 240, G^ are very simple, andlie in the loop of the intestine. The ovary is the larger,an4 contains a cavity into which the ova are set free, andfrom which they pass outwards along an oviduct whichopens into the cloacal chamber. The testis surrounds theovary, and is mature at a different time (dichogamy); its29 450 SUB-PHYLUM UROCHORDA OR TUNICATA. duct runs by the side of the oviduct. In some forms, wherethe gonads are near the cloaca, there are no ducts. Theova are surrounded by follicular cells, and probab
. Outlines of zoology. stem.—Tunicates are reproductive organs (Fig. 240, G^ are very simple, andlie in the loop of the intestine. The ovary is the larger,an4 contains a cavity into which the ova are set free, andfrom which they pass outwards along an oviduct whichopens into the cloacal chamber. The testis surrounds theovary, and is mature at a different time (dichogamy); its29 450 SUB-PHYLUM UROCHORDA OR TUNICATA. duct runs by the side of the oviduct. In some forms, wherethe gonads are near the cloaca, there are no ducts. Theova are surrounded by follicular cells, and probably fertilisedin the cloaca. Development.—The fertilised ovum divides completely and almostequally. The spherical blastosphere becomes slightly flattened, andultimately forms a two-layered gastrula. Along the dorsal median line of the gastrula the ectoderm cells formthe medullary groove, the sides of which arch together and form acanal—the medullary canal. This opens anteriorly to the exterior by NC. ? NP E NCH-. Fig. 242.—Young embryo of Ascidian (Clavelind).—AfterVan Beneden and Julin. NF., Neuropore ; NC, neural canal; NCH., notochord ; E.,ectoderm ; M., mesoderm; A., archeuteron. the neuropore, and posteriorly communicates with the archenteron bvthe neurenteric canal. With regard to the origin of mesoblast and notochord, there is moredifficulty. Both originate from the endoderm in the region of theblastopore, and for a time grow forward together. The notochord liesin its usual position on the roof of the gut, from a specialisation ofwhich it arises; but its forward extension is limited,—it never extendsinto the anterior region, and in the posterior region—the future tail—itincreases at the expense of the primitive gut, whose lumen it mesoderm, on the other hand, extends right forward, and becomesdivided into two regions—a posterior, ultimately forming the muscula-ture of the tail, and an anterior, giving rise to the blood, connectivetissues,
Size: 1719px × 1453px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1910, bookidcu3192, booksubjectzoology
