The encyclopædia britannica; a dictionary of arts, sciences, literature and general information . Castle also discusses some important theoretical questions. Hepoints out that, in Ciona at least, the chorda-mesenchyme ring takes Eart along with endoderm in the primary invagination, and soelongs to the primary endoderm; while the rest of the mesoderm,the muscle cells of the neuro muscular ring, are carried in by asecondary invagination, and belong to the outer layer of the younggastrula, or primary ectoderm. He considers that the chorda mustbe regarded as a mesodermal organ. He agrees with form
The encyclopædia britannica; a dictionary of arts, sciences, literature and general information . Castle also discusses some important theoretical questions. Hepoints out that, in Ciona at least, the chorda-mesenchyme ring takes Eart along with endoderm in the primary invagination, and soelongs to the primary endoderm; while the rest of the mesoderm,the muscle cells of the neuro muscular ring, are carried in by asecondary invagination, and belong to the outer layer of the younggastrula, or primary ectoderm. He considers that the chorda mustbe regarded as a mesodermal organ. He agrees with former obser-vers m seeing no trace of enterocoele formation, and he doubtswhether any Chordata are Enterocoela. He does not believe indistinguishing those Metazoa with a mesoderm from those with a mesenchyme. He considers that embryology gives no supportto the Annelid hypothesis as to the origin of Chordates. A long-continued discussion as to the origin, nature and fate ofcertain cells, the testa-zellen, which make their appearancebetween the young embryo and its follicle (fig. 12), has ended in. * For structure of other forms, see below. For reproduction by gemmation see under Classificationbelow. (Alter PUon.) Flc. 12.—Portion of Mature Cvum of Ascidian, showingF, follicle, and/, r, test-cells. practical agreement that these small cells are derived from thefollicle-cells, and have nothing to do with the test. In Salpa, how-ever, certain follicle-cells enter the embryo, and perform importantfunctions in guiding the development for a time. In most Ascidians the eggs are fertilized in the peribranchialcavity, and undergo most of their development before leaving theparent; in some cases, however, the eggs are laid, ^^^^ takes place in the surrounding water. Thesegmentation is complete and regular (ng. 13, A) and results inthe formation of a spherical blastula, which then undergoesinvagination (fig. 13, B). The embryo elongates, and the blastoporeor invagin
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectencyclo, bookyear1910
