188 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 à 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. 
III". The processes affecting thickness of the layer are of very com- 
mon occurrence, and are of two kinds, — (4) processes leading to an 
inerease, and (5) those leadin 
variations in thickness may be general, extending over a considerable 
g to a diminution in thickness. These 
part or the whole of the area under consideration, or they may be 
restricted to a small part of that area. 
Nerve cora. 
^ 
Mesoder, 
LS 
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 atrarius) to 
show the linear thickening of the ectoderm to produce the nerve cord. (From H. 
V. Wilson, "91, Bull. U. S, Fish Commission, IX. Plate XCV.) 
