. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. DETERMINATION IN SABELLARIA 207 oesophagus at all. On the ventral surface of the larva is a long, conical outgrowth, completely covered with long actively-moving cilia which are characteristic of the oesophagus. DISCUSSION OF RESULTS There are two important points of difference between larvae which develop from eggs without membranes and normal larvae. The first is the complete absence of an internal gut and the second the striking deviation from the normal form of the larva. Instead of possessing the fairly spherical shape,
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. DETERMINATION IN SABELLARIA 207 oesophagus at all. On the ventral surface of the larva is a long, conical outgrowth, completely covered with long actively-moving cilia which are characteristic of the oesophagus. DISCUSSION OF RESULTS There are two important points of difference between larvae which develop from eggs without membranes and normal larvae. The first is the complete absence of an internal gut and the second the striking deviation from the normal form of the larva. Instead of possessing the fairly spherical shape, they have elongate bodies in which the posterior structures, the posterior cilium and the paired bristles, are dorsally displaced. The distortion of the posterior part of the larva may be accounted for if we assume that the endoderm cells, which differentiate normally inside the ectoderm to form the gut, grow out so that they come to lie on the surface of the B FIG. 6. Partial exogastrulae. Seventy-two hours after fertilization. A. Dorsal view. The opening to the stomach, the stomach, and the intestine are visible. B. Ventral view. The bristles have been omitted in the drawing; only their points of origin in the chsetae-sacs are indicated. The opening to the stomach may be seen at the base of the long conical projection covered with short cilia. That this posterior outgrowth of the endoderm cells has indeed occurred is shown by the following considerations: 1. The gut is com- pletely lacking in these larvae, and within the post-trochal region of the larva there is a large cavity. 2. The cells situated at the posterior end of these larvae are not pigmented and never develop chromato- phores. In the normal larva the chromatophores are found in the ectoderm; only the endoderm is colorless. 3. In the normal larva, motile cilia do not appear on the external surface of the post-trochal region until approximately thirty-five hours after fertilization. \\ hen they do develop,
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Keywords: ., bookauthorlilliefrankrat, booksubjectbiology, booksubjectzoology
