256 



Embryogenesis: Progressive Differentiation 



inductive action on the part of the lateral 

 wall of the rhombencephalon. In the case 

 of the lens, the optic vesicle provides the 

 essential inductive stimulation in most am- 

 phibian species, but in at least some species 

 it is preceded by a conditioning infkience of 

 the head mesoderm. The latter may be suffi- 

 cient to call forth a lens in those species 

 which can form it in the absence of an optic 

 vesicle. Probably both the rostral part of the 

 archenteron roof and the prospective fore- 

 brain are involved in the induction of the 

 nasal placode, and both head mesectoderm 

 and archenteron roof derivatives are con- 

 cerned with balancer induction. In the case 

 of the pituitary gland, the invagination of 

 the ectoderm seems to be induced by oral 

 ento-mesoderm, while its final differentiation 

 depends upon a subsequent contact of this 

 ectoderm with the infundibulum. 



It has been shown in several of these in- 

 stances that in the absence of one of the two 

 components, the induced structure fails to 

 attain normal shape and differentiation. 

 When an inductive substratum is deranged, 

 or when its contact area with the ectoderm 

 is abnormally large, double or multiple 

 formations may result, such as supernumer- 

 ary nasal epithelia (Holtfreter, '36) or 

 medullary plates (Waddington, '40). In order 

 to call forth normal structures, the con- 

 stituents of the "inductor systems" must 

 be arranged typically with respect to the 

 ectoderm, and they must act at the proper 

 time. 



To complete this brief svirvey of ectodermal 

 inductions we merely enumerate others: epi- 

 dermal glands, enamel organs, gills, and fin. 

 In addition, the characteristic perforations 

 which produce mouth opening, gill slits, and 

 anus have sometimes been listed under the 

 term "induction." It is true that they result 

 from a contact action between specific parts 

 of the intestinal tract and the overlying ecto- 

 derm. Yet, the physiological processes mak- 

 ing for the thinning out and regression of 

 these ento-ectodermal membranes appear to 

 be quite different from those which cause the 

 cytological determination of the other struc- 

 tures dealt with above. 



HOMOIOGENETIC INDUCTION 



As pointed out above, certain parts of the 

 nervous system are engaged in the induction 

 of ectodermal organs (nose, lens, otocyst, 

 pituitary). On the other hand, in the early 

 gastrula only the chorda-mesoderm has in- 

 ductive capacity, whereas the ectoderm lacks 



it. Obviously, the medullary portion of the 

 ectoderm becomes inductive in postgastrula 

 stages. Mangold and Spemann ('27) dis- 

 covered that pieces of the medullary plate of 

 urodeles when transplanted into the blasto- 

 coele of young gastrulae would induce a 

 secondary medullary plate in the host, and 

 Mangold ('29a) showed that all parts of the 

 plate acquire the capacities for self-differen- 

 tiation and induction simultaneously. The 

 authors used the term "homoio genetic in- 

 duction'" for this phenomenon. In accordance 

 with the regional differences in the inductive 

 specificity of the archenteron roof, anterior 

 parts of the plate induce mainly brain and 

 sense organs; middle portions, a spinal cord; 

 and posterior parts, various tail structures 

 (Mangold, '29a, '32, '33b; Bytinski-Salz, '29; 

 Ter Horst, '48). 



The experimental conditions leading to the 

 manifestation of homoiogenetic induction are 

 entirely artificial since, normally, the neviral 

 plate has no chance to act upon gastrula ecto- 

 derm. From this viewpoint, the phenomenon 

 belongs in the category of "abnormal in- 

 ductions" (see p. 267). However, it is likely 

 that the agents which neuralize gastrula ecto- 

 derm are identical with those which are 

 instrumental in the normally occurring in- 

 duction of the above-mentioned placodal 

 structures, all of which differentiate from 

 neurula and tail-bvid epidermis, and that the 

 differences in response are due to changes 

 in the reaction potency (competence) of the 

 ectoderm. In fact, it has been shown that the 

 ectoderm loses its neural competence towards 

 the end of gastrulation, while acquiring 

 local competences for nasal epithelium, oto- 

 cyst, and so forth. If the above interpretation 

 is correct, then the seemingly atypical phe- 

 nomenon of "homoiogenetic" neural induc- 

 tion would be merely an expression of the 

 age differences between inductor and react- 

 ing tissue. 



CHAINS OF INDUCTION 



Tissues such as the neural plate which 

 must first be induced in order to become 

 capable of inducing other structures have 

 been termed "secondary inductors." Subse- 

 quently, the secondarily induced structures 

 may act as "tertiary inductors": the otocyst 

 induces the surrounding mesenchyme to 

 form a cartilaginous capsule, and it appears 

 that the lens stimulates the overlying ecto- 

 derm to adopt the characteristics of a cornea. 

 Thus differentiation, particularly of the ecto- 

 dermal structures, proceeds through the 



