262 



Embryogenesis: Progressive Differentiation 



tained with Triturus liver which are based 

 upon a statistically siifficient number of 

 cases (185). As far as the host influence is 

 concerned, the extensive data on mouse 

 kidney gave much the same results. In 

 Figure 90 the distribution of the various in- 

 duced structures over the host is plotted 

 according to four cephalocaudal levels. No 

 distinction was made between direct and 

 indirect inductions; for instance, the listed 

 ear vesicles could have been induced directly 

 by the graft or indirectly by hindbrain which 

 was induced first. 



ways. Unfortunately, the inductive specificity 

 of these tissues had not been tested in ecto- 

 derm explants. Nevertheless the diagram of 

 Figure 91 shows again that cephalic in- 

 ductions tend to increase toward the head 

 region, and trunk-tail inductions toward the 

 tail region of the host. Once more, otocyst 

 and balancer fall out of line, having their 

 maximum of occurrence not in the head but 

 in the anterior trunk region. 



The available evidence indicates that there 

 does exist a specifying influence of the host 

 level upon accessory inductions, although 



;vH 





/^ 



Tig. 95. Fig. 96. 



Fig. 95. Induction by fresh liver of Triturus implanted in ectodermal vesicle of Triturus alpestris. Notice tail 



and balancer. (From Chuang, '38.) 



r^'g. 96. Induction by fresh liver of Triturus, implanted in ectodermal vesicle of Triturus alpestris (see Fig. 



95). Notice spinal cord, notochord, somites, tail fins. (From Chuang, '38.) 



If one uses the rate of incidence of certain 

 inductions in explants as a control (right 

 ordinate), it is apparent that host influences 

 reinforce the appearance of a given struc- 

 ture in certain levels and redvice the chance 

 of its occurrence in other levels. Brain, nose 

 and eye inductions clearly decrease in a 

 cephalocaudal direction while those of pro- 

 nephros, muscle and tail decrease in the 

 opposite direction. However, as was the case 

 in explants, head and tail structures can be 

 induced simultaneously at any host level. 

 Apparently, specific effects of the graft and 

 regional host influences interact and compete 

 with each other in a complicated way. The 

 former may account for the rather unex- 

 pected frequency of otocysts and balancers in 

 trunk levels. 



Strikingly similar distribution patterns of 

 specific inductions within a host were ob- 

 served by Holtfreter ('34b) and Toivonen 

 ('40), who used a great variety of adult tis- 

 sues which had been pretreated in different 



this influence asserts itself only in a certain 

 percentage of cases. This phenomenon does 

 not seem to be covered by our explanations 

 given above and one has to look for other 

 possible interpretations. The following sug- 

 gestions have been made: 



(1) The head mesoderm differs from trunk 

 mesoderm quantitatively and possibly also in 

 its reaction potency (Chuang, '39) ; hence 

 the difficulty of the grafts to induce mesoder- 

 mal trunk structures in the head region. (2) 

 The competence of the ectoderm appears to 

 vary along the cephalocaudal axis (Spemann, 

 '31). This point has been discussed before 

 (p. 258). Further indications for such a 

 gradient are found in experiments which 

 show that the ectodermal response to a graft 

 is usually more pronounced in the anterior 

 than in the posterior ventral regions of the 

 embryo (Lehmann, '32; Machemer, '32; 

 Holtfreter, '34b; Schechtman, '38a; Chuang, 

 '39). (3) The primary host inductors may 

 tend to assimilate the accessorily induced 



