. The Cambridge natural history. Zoology. DEVELOPMENT 173. ment takes place in a screw line; when this ceases the larva rests on its hinder pole, and the cilia cause it to turn round on its axis. Sections show that the larva is built up of two layers: 1. " The inner mass/' consisting of various kinds of cells in a gelatinous matrix. 2. A high flagellated epithelium, which entirely covers the larva with the exception of the hinder pole. The cells in the inner mass are classified into (1) undifferenti- ated cells, recognised by their nucleus, which possesses a nucleolus ; these are the arch
. The Cambridge natural history. Zoology. DEVELOPMENT 173. ment takes place in a screw line; when this ceases the larva rests on its hinder pole, and the cilia cause it to turn round on its axis. Sections show that the larva is built up of two layers: 1. " The inner mass/' consisting of various kinds of cells in a gelatinous matrix. 2. A high flagellated epithelium, which entirely covers the larva with the exception of the hinder pole. The cells in the inner mass are classified into (1) undifferenti- ated cells, recognised by their nucleus, which possesses a nucleolus ; these are the archaeo- cytes; (2) differentiated cells, of which the nucleus contains a chromatin net; these give rise to pinaco- cytes, coUencytes, and scleroblasts. Some of them form a flat epithelium, ^^'^- 68.âLongitudinal section through the hinder â u. 1 j_i 1 â 1 Pols of the larva of G. varius. a' Flagellated which covers the hmder cells ; rm,\ undifferentiated cell; ,nd\ differen- pole. Some of the SClero- *'^''''^ '^^^^ > -^'â ' P'gâ¢ent; x, surface of hinder f pole. (After Maas.) blasts already contain spicules. Fixation, occurs very early. The front pole is used for attachment, the pigmented pole becoming the distal end (Fig. 69). The larva flattens out, the margin of the attached end is produced into radiating pseudopodial processes. The flagellated cells retreat to the interior, leaving the inner mass exposed, and some of its cells thereupon form a flat outer epithelium. This is the most important process of the meta- morphosis ; it is followed by a pause in the outward changes, coinciding in time with rearrangements of the internal cells to give rise to the canal system; that is to say, lacunae arise in the inner mass, pinacocytes pass to the surface of the lacunae, and form their lining; the flagellated cells, which have lain in con- fusion, become grouped in small clusters. These become flagellated chambers, communications are established between the various portions of
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1895
