. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology; Zoology. 188 bulletin: museum of comparative zoology. Typical linear folds occur iu the formation of the neural tube of most Vertebrates, of the lateral line of fishes, and of the atrium of Amphioxus. (K. & H., Fig. 881.) 3. Inequalities of growth in different parts of the layer may take place, so that parts originally lying in a line move out of that line. By this means are produced folds lying in the plane of the layer, — folds com- parable to those made by an advancing wave front on entering a bay. Such folds m
. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology; Zoology. 188 bulletin: museum of comparative zoology. Typical linear folds occur iu the formation of the neural tube of most Vertebrates, of the lateral line of fishes, and of the atrium of Amphioxus. (K. & H., Fig. 881.) 3. Inequalities of growth in different parts of the layer may take place, so that parts originally lying in a line move out of that line. By this means are produced folds lying in the plane of the layer, — folds com- parable to those made by an advancing wave front on entering a bay. Such folds may occur in the bounding line of the growing layer, as happens, it is alleged by some, in the overgrowth of the yolk by the blastoderm, — a process by which the crescentic notch is produced on the edge of the blastoderm. Again, such folds may occur in the midst of a layer, pro- ducing a relative displacement of the points lying in that layer. To this process may be referred the remarkable rotation of the gill slits and mouth of Amphioxus,—the gill slits rotating from the right to the left side of the larva, the mouth from the left side towards the mid- ventral line. IIP. The processes affecting thickness of the layer are of very com- mon occurrence, and are of two kinds, — (4) processes leading to an increase, and (5) those leading to a diminution in thickness. These variations in thickness may be general, extending over a considerable part or the whole of the area under consideration, or they may be restricted to a small part of that area. -Ate r Fig. 20. 4. The process of thickening is illustrated in the lateral walls of the neural tube of Vertebrates (Figure 19), and, localized, in the formation of the neural tube in Teleosts (Figure 20). Other examples are seen in Fig. 20. Cross section of the blastoderm of a bony fish (Serranus atrarins) to show the linear thickening of the ectoderm to produce the nerve cord. (From H. V. Wilson, '91, Bull. U. S. Fish Comm
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