II. THE PERIOD OF ORGAN DEVELOPMENT 139 



the rostral ectoderm. A mouth plate, consisting of ectoderm 

 and endoderm is thereby formed. Later this breaks through, 

 and a mouth is formed. This process is due to induction by the 

 endoderm of the archenteron wall. If, at the neural plate stage, 

 this endoderm is grafted somewhere under the ectoderm of 

 the flank, a mouth cavity will be formed at this point (Stroer, 

 1933). Balinsky (1948) has demonstrated that here, too, a 

 weak predetermination precedes the final determination of the 

 mouth. Probably this predetermination takes place during 

 gastrulation. The results of xenoplastic transplantations are 

 highly interesting. In urodeles (newts), the larvae have real 

 teeth, consisting of dentine. In the anurans (tail-less amphi- 

 bians), however, they have horny teeth, formed by local 

 proliferation and cornification of the mouth epithelium, and 

 the mouth edges are covered with horny jaws. Spemann and 

 Schotte (1932) grafted belly ectoderm of anurans into the ventral 

 side of the head of Triton. This ectoderm reacted to the inductive 

 activity of the Triton mouth endoderm, with which it came 

 into contact, by forming a mouth cavity. This, however, had 

 the character of an anuran mouth, and was provided with horny 

 jaws and teeth. Moreover, immediately behind the mouth, a 

 pair of suckers, consisting of glandular epithelium, developed 

 from the grafted anuran ectoderm. These organs are always 

 found in the corresponding place in anuran larvae, but the 

 urodeles do not have them (PI. X). The tissue of the Triton 

 host, therefore, had induced organs in the graft which the host 

 itself did not possess. Holtf refer (1935) made the reverse 

 experiment, which had the expected result. Belly ectoderm 

 of Triton, grafted into the head of an anuran embryo, produced 

 a mouth with dentine teeth. These experiments give a very 

 clear illustration of the nature of induction. The cells of the 

 graft react to an "order" given by the inductor, but they do 

 it in their own way, by virtue of their own species- and organ- 

 specific reactive powers, which are due to their genetic con- 

 stitution, and their previous chemodifferentiation. Induction 

 does nothing but activate a definite developmental capacity, 

 present in the cell, and dependent on the nature of the latter. 

 Somewhat more caudally, a row of lateral extrusions, the 



