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bulletin: museum of comparative zoology. 



Typical linear folds occur in 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 yolli 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. 



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 fisli (Serranus atrarius) to 

 sliow the linear thickenins: of the ectoderm to produce the nerve cord. (From H. 

 V. Wilson, '91, Bull. U. S. Fish Commission, IX. Plate XCV.) 



