246 



Embryogenesis: Progressive Differentiation 



a one-to-one relationship between a particu- 

 lar induced structure and a histologically 

 defined component of the inducing system. 

 This, however, has not been possible, partly 

 because of the tendency of grafted fragments 

 of the organizer to regulate into a variety of 

 tissues, and partly because of the fact that 

 what is induced is not a rigid mosaic of tis- 



a b 



Fig. 83. a. Induction of ectodermal head structures 

 by "head organizer" which was grafted into isolated 

 ectoderm, h. Induction of trunk and tail structures 

 by "trunk-tail organizer." (After Holtfreter, '36.) 



sue primordia but a pattern of overlapping 

 new embryonic fields which then undergo 

 self-organization into definite organs and 

 tissues. But although the regional differences 

 in inductivity have eluded the attempts to 

 localize them precisely, their existence can- 

 not be denied. 



The first attempt at demarcating the area 

 capable of causing inductions was made by 

 Bautzmann ('26). He found that in early 

 Triturus gastrulae the material capable of 

 inducing a neural tube is confined to the 

 quadrant above the dorsal blastoporal lip, 

 a region which in isolation undergoes pro- 

 nounced self-organization. In anurans, the 

 neuralizing material appears to be restricted 

 to a narrower dorsal sector, since in at least 

 four genera studied for this purpose (Rana, 

 Hyla, Discoglossus, Bufo) the neuralizing 

 material extended to about 45 degrees on 

 either side of the dorsal mid-line, as against 

 90 degrees in Triturus (Schmidt, '36; 

 Schechtman, '38b; Raunich, '40). Grafts from 

 more lateroventral sectors of the marginal 

 zone of the anuran gastrula could induce a 

 tail which frequently lacked a spinal cord 

 but contained the induced derivatives of a 

 neural crest, such as mesenchyme and chro- 

 matophores. 



This method of testing the induction power 

 of parts of the marginal zone by implanting 

 them into another gastrula gave somewhat 

 ambiguous results because of an occasional 



interference of determining influences of the 

 host (p. 259). Other methods circumvent this 

 source of complication. A clear specificity of 

 the organizer regions was foiuid when either 

 the anterior or posterior parts of the prospec- 

 tive archenteron roof were grafted xenoplas- 

 tically into a bag of gastrula ectoderm iso- 

 lated from the animal pole region (Holt- 

 freter, '36). The anterior part regularly in- 

 duced head structures: brain, sense organs, 

 visceral cartilage and mouth structures; the 

 posterior part always induced trunk struc- 

 tures: spinal cord and, less frequently, so- 

 mites, notochord and pronephros, which 

 tended to grow out into a tail (Fig. 83). 

 Mesenchyme and chromatophores could be 

 induced by either of the grafts. Fvirther evi- 

 dence of inductive differences along the 

 cephalocaudal extent of the archenteron roof 

 could be derived from the partial exogas- 

 trulae (p. 236). When gastrula ectoderm was 

 placed upon the exogastrulated endomeso- 

 derm, the cephalic portion of the latter in- 

 duced brain structures, and the trunk portion 

 a tail (Holtfreter, '33d). 



More detailed information on the distribu- 

 tion of inductive cell material in the early 

 gastrula has been furnished by the afore- 

 mentioned isolation experiments (p. 238). 

 The explants which comprised small sectors 

 of the marginal zone frequently developed 

 some ectoderm which either came from the 



Fig. 84. a. The location of the "head-organizer" 

 (dots) and of the "trunk-tail organizer" (dashes) is 

 indicated on the side view of an early urodele gas- 

 trula. h. Dorsal view of a gastrula; distribution of 

 ears (dots), noses (circles) and eyes (circles with 

 dots), as obtained in small explants from this area. 

 (After Holtfreter, '38b.) 



prospective ectoderm that was included in 

 the explant, or had been formed by regu- 

 lation from prospective chorda-mesoderm. 

 As is indicated in Fig. 84a!, cephalic in- 

 ductions occurred associated with head meso- 

 derm and the anterior portion of notochord 

 and somites, while spinal cord and tail fin 

 differentiated in association with the more 

 posterior portion of notochord and somites 



