Fig. 7.

Tensile and compressive forces in development. (A) Fig. 44 in His (1875) showing the formation of a split in the ventral germ layer (Dd) of a bilaminar chicken embryo, resulting from the lateral tension produced by the faster growth of the upper layer (pp. 58–59). The loose cells torn from the lower layer will become the lower leaf of the mesoderm. oG, epiblast; Z, cell stretched across the gap. (B) Fig. 10 in His (1894). Sheet of clay with a sudden transition (arrow) from thick (8 mm) to thin (4 mm). In the lower illustration there is a span of 15 cm, but in the upper illustration the sheet has been pushed from the left to decrease the span to 12 cm. The result is an asymmetric fold resembling the head fold in C. (C) Fig. 26 in His (1894), ray (Torpedo sp.) embryo showing the head fold. Original rotated 90° anticlockwise and horizontally reflected by us. (D) Fig. 17 in His (1894; also reproduced as fig. 3 in Hopwood, 1999). A strip of leather, forced into a curve and fixed along one side. The resultant forces throw the leather into folds resembling intersomitic clefts.