II. THE PERIOD OF ORGAN DEVELOPMENT 



141 



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Fig. 51. (a) Embryo of Triton cristatus, into which a piece of pro- 

 spective ectoderm of T. taeniatus has been grafted at the gastrula 

 stage (cf. Pig. 39b). The graft occupies the prospective gill area 

 of the right side, (b) Dorsal view of the same embryo at a later 

 stage; development has proceeded farther in the gills on the right, 

 formed by the taeniatus ectoderm, than in those on the left (formed 

 by the host), (c) Cross section through the gill region of the embryo 

 (b) (the gill primordium formed by the graft is on the left in this 



figure). After Spemann. 



Very many experiments have been made in connection with 

 the limbs of amphibians. Their first visible primordia are 

 formed by the local accumulation, under the ectoderm of the 

 flank, of mesenchyme originating from the lateral mesoderm. 

 First, a half-spherical limb-bud is formed. This then assumes a 

 conical shape, and subsequently begins to expand at its tip, 

 thereby forming the hand- (or foot-) plate. The digits are 

 formed by indentation of the edge of this plate, and become 

 elongated in a certain sequence. Still later the joints are form- 

 ed, and the limb is rotated so that it assumes the normal 

 position relative to the body. Meanwhile, the skeleton and the 

 musculature of the limb have differentiated from the mesen- 



