258 



Embryogenesis: Progressive Differentiation 



an early gastrula, the induced medullary 

 plate appears synchronously with that of the 

 host. Assuming that these grafts begin to re- 

 lease the neuralizing agent immediately 

 after implantation, a precocious medullary 

 plate might have been expected. Since this 

 did not happen, it has been inferred that the 

 ectoderm acquires neural competence not 

 before the middle or late gastrula stages. 

 In view of the following data, the interpreta- 



nized with these two inductive events are 

 successive states of competence: the ecto- 

 derm of the gastrula and early neurula will 

 respond only to the mesodermal inductor, 

 but is refractory to the neural inductor. 

 During neurulation, the ectoderm acquires 

 the capacity to form otocyst in response to the 

 neural inductor while gradually losing re- 

 sponsiveness to the mesodermal inductor 

 (Fig. 89). 



MESODERMAL NEURAL 



DIFFERENTIATION 

 OF EAR ECTODERK 



Stage 



Fig. 89. Graphic representation of relative intensities of various components of ear inductions in Ambly- 

 stoma. The maxima are placed at the same level arbitrarily. Abscissa: Harrison stages (12 = small yolk plug; 

 32 1= advanced tail bud). Ordinate: Degree of ear differentiation in arbitrary units. (From Yntema, '50.) 



tion may be somewhat different. Neuraliza- 

 tion could be produced in the ectoderm of 

 early gastrulae by exposing it for only a few 

 minutes to cytolyzing solutions (p. 272). 

 Nevertheless, the induced neural plate ap- 

 peared synchronously with that of the donors 

 of the ectoderm. Evidently, the early gastrula 

 ectoderm is already competent for neuraliza- 

 tion, but there was a latent period between 

 the time of stimulation and visible differen- 

 tiation. 



The analysis of otocyst induction by 

 Yntema ('50) provides a good example of the 

 synchronization of competence and induc- 

 tions in the normal embryo. Unspecified 

 gastrula ectoderm, or prospective gill ecto- 

 derm of older stages, was transplanted to 

 the ear region, including a wide variety of 

 donor and host stages in altogether more 

 than 100 combinations. It has been known 

 that both mesoderm and prospective hind- 

 brain are instrumental in otocyst induction 

 (see Harrison, '45). Yntema showed that the 

 mesodermal action which occurs first is 

 maximally effective in early neurula stages, 

 whereas the hindbrain operates somewhat 

 later with a maximum of activity in the 

 early tail-bud stages. Remarkably synchro- 



REGIONAL PATTERNS OF COMPETENCE 



The prospective ectoderm of the early gas- 

 trula shows no regional differences in neural 

 or other competences (see p. 273), but such 

 differences do develop gradually in the em- 

 bryo. For instance, in the late neurula and 

 early tail-bud stages, the lens-competence of 

 the ectoderm is in some species narrowly 

 restricted to the normal lens-forming area 

 and its adjacent ectoderm; in Bombinator 

 pachypus it is limited to the head epidermis, 

 and in a number of species of Rana and 

 Triturus it extends over the entire head 

 and trunk (reviews of this much-discussed 

 problem in Mangold, '31a; Spemann, '38; 

 Needham, '42). In the same stages, the com- 

 petence for balancer formation in A. puncta- 

 tum is confined to the balancer-forming epi- 

 dermis and its immediate neighborhood 

 (Harrison, '25). Similar studies have been 

 made with regard to the competence for 

 mouth and gill formation, but the exact pat- 

 tern of these competences has not been in- 

 vestigated, and the suggestion of Spemann 

 ('12) that the reaction potencies for dif- 

 ferent structures may represent overlapping 

 gradient fields {"Reaktionsfeld" Mangold, 



