AMPHIBIANS 



245 



host tissues tend to deviate the gastrulation 

 movements of the graft, carrying the latter 

 along with their own cell streams. 



As a rule, the induced mesodermal tissues 

 seem to result from a conversion of the host's 

 ventrolateral mesoderm, although their deri- 

 vation from the pluripotential ectoderm is 

 within the range of possibilities. The pro- 

 spective entoderm apparently cannot be in- 

 duced to form mesodermal or ectodermal 

 tissues. However, the blastoporal graft may 

 affect the subjacent host entoderm insofar as 

 the latter frequently forms a cavity, resem- 

 bling that of an archenteron, which follows 

 the length of the grafted notochord. The 

 cavity later becomes confluent with the host's 

 gut lumen. There is no evidence to show that 

 the affected entoderm differentiates contrary 

 to its prospective fate. If induction is defined 

 as a process which determines primarily the 

 cytological fate of the reacting cells, then 

 this entodermal reaction would not fall un- 

 der the term since it involves merely the cell 

 arrangement and not the trend of differen- 

 tiation of the entoderm. 



The accessory medullary plate folds inward 

 synchronously with that of the host. If the 

 graft is sufficiently large, a correspondingly 

 large plate will be induced which may de- 

 velop into a complete neural system com- 

 prising the various brain portions, sense or- 

 gans, spinal cord, and neural crest deriva- 

 tives. The organization of the secondary em- 

 bryo may be completed by the appearance of 

 typically located gills, limbs, dorsal fin, 

 balancer, teeth and other ectodermal deriva- 

 tives (Holtfreter, '33d; Ekman, '37; Nieuw- 

 koop, '47). However, in most of the reported 

 cases only parts of the neural system were 

 induced (Figs. 92a, b), svich as a brain of re- 

 duced size which usually showed defects of 

 various degrees. The incomplete brains were 

 bilaterally symmetrical and they differenti- 

 ated into the tracts and nuclei which are 

 characteristic of the homologous normal 

 brain divisions (Nieuwkoop, '47). 



Together with these morphological char- 

 acteristics, the secondary embryo appears to 

 acquire the biochemical mechanisms neces- 

 sary for its normal physiological functions. 

 The muscles of the accessory head and trunk 

 become innervated and exhibit rhythmical 

 contractions, either spontaneously or in re- 

 sponse to external stimulation. This occurs 

 likewise if the inductions have been pro- 

 duced in an isolated piece of ectoderm, show- 

 ing that the muscle innervation is actually 

 supplied by the induced neural system (Holt- 



freter, '36). In both the normal and the ac- 

 cessory embryo, the transformation of the 

 medullary plate into a tube is a matter of 

 cellular shiftings and changes of cell shape 

 rather than of differential growth. Subse- 

 quently not only the normal neural tube 

 (Burt, '43) but also neural tissue induced by 

 chemical substances (Waddington, '40) show 

 an intense mitotic activity. The cholinesterase 

 activity of the experimentally induced neural 

 structures is of the same order and magni- 

 tude as that of the primary nervous system 

 of the host (Boell and Shen, '44). 



In conclusion, the grafted blastoporal ma- 

 terial has demonstrated the capacity to in- 

 duce all the ectodermal structures of a nor- 

 mal embryo and, in addition, to supply the 

 mesodermal tissues of an axial system, partly 

 out of its own substance and partly by re- 

 cruiting them inductively from the host. 

 Taken together with the experimental evi- 

 dence from our preceding chapters these ob- 

 servations leave no doubt that the organizing 

 mechanisms here disclosed are likewise in- 

 strumental in the normal morphogenesis of 

 the amphibian embryo. Comparative studies 

 on the organizer activity in different am- 

 phibian species did not reveal any marked 

 differences; in fact, the essential features 

 here described on the basis of experiments 

 on urodeles are also found in various anuran 

 species (Geinitz, '25a; Bytinski-Salz, '29; 

 Schotte, '30; Dalcq, '33, '40, '43; Holtfreter, 

 '35a; Pasteels, '40, '45; Schechtman, '38a,b; 

 Raunich, '40). 



DISTRIBUTION OF REGIONALLY 



SPECIFIC INDUCTORS IN THE 



EARLY GASTRULA 



The formation of a secondary embryo in- 

 volves an axial self-organization of the blas- 

 toporal graft as well as an axial patterning 

 of the induced structures. Brain tissues ap- 

 pear normally in connection with the an- 

 terior portion of the invaginated archenteron 

 roof and tail tissues with the posterior por- 

 tion. That this patterning of the inductions is 

 not due to a regionally different responsive- 

 ness of the ectoderm to a general stimulus 

 became clear when it was found that either 

 head or tail structures can be readily in- 

 duced from any portion of the early ecto- 

 derm. Evidently, what has been provision- 

 ally called "the organizer" must be sub- 

 divided into regional components with spe- 

 cific capabilities of differentiation and in- 

 duction. It would seem desirable to establish 



