250 



Embryogenesis: Progressive Differentiation 



'41; Chuang, '47). This restriction of poten- 

 tialities of the caudal primordia is the more 

 remarkable as in larval stages it is the tail 

 and not the trunk which is outstanding for 

 its capacity of regeneration. Hence, one can 

 no longer subscribe to the notion that the 

 determination of the axial organs proceeds 

 along a cephalocaudal gradient. 



The different tissue primordia of the tail, 

 namely notochord, spinal cord, somites and 

 fin, are each independently capable of axial 

 stretching. But it seems that the cooperation 

 of several, or all, of them is necessary for the 

 formation of a complete and straight tail 

 (Vogt, '26b; Holtfreter, '33c; Bytinski-Salz, 

 '36; Chuang, '47; Kitchin, '49).* 



REGIONAL INDUCTION CAPACITIES OF 

 THE MESODERM OF THE NEURULA 



We saw that even in the young gastrula a 

 distinction can be made between head and 

 trunk-tail organizers, although their borders 

 are ill-defined. At the end of gastrulation, a 

 more distinct localization of the various 

 mesodermal inductors along the cephalo- 

 caudal axis is manifested. The axial meso- 

 derm has meanwhile separated into a poste- 

 rior portion (notochord and somites) and an 

 anterior one (prechordal mesoderm). The 

 latter begins at the level of the midbrain, 

 between the first and second visceral arch. 

 It is subdivided into a narrow median strip, 

 the "prechordal plate," and the lateral "man- 

 dibular mesoderm" whose paraxial position 

 corresponds to that of the trunk somites 

 (Vogt, '29; Adelmann, '32; and others; see 

 Fig. 85). Lehmann ('42b, '45, '48) has pointed 

 out that this mesodermal structuration is re- 

 flected in differences of inductive capacity. 

 He considers the prechordal mesoderm as a 

 specific inductor for tel- and diencephalon 

 which are designated as "archencephalic" 

 structures, whereas the chordal-parachordal 

 head mesoderm would induce posterior or 

 "deuterencephalic" brain structures. Dalcq 

 ('46) prefers the terms "acrencephalic" and 

 "chordencephalic" inductors. No structural 

 difference can be detected between the latter 

 and the spinal cord-inducing trrmk portion 

 of the archenteron roof. 



In order to examine possible regional dif- 



* This balance between independence and cooper- 

 ation of the components of the tail is nicely demon- 

 strated in L. S. Stone's time-lapse film of the de- 

 velopment of A. punctatum. The speeded-up pic- 

 tures show an up-and-down wiggling of the elongat- 

 ing tail bud, which seems to reflect alternate spurts 

 of stretching in the ventral and dorsal tail portions, 

 respectively. 



ferences in the inductive capacity of the 

 archenteron roof. Mangold ('33b) divided 

 the latter into four transverse strips and 

 implanted each under the ectoderm of a 

 gastrula. Grafts of the rostral strip were 

 relatively inactive, possibly because they 

 consisted largely of cephalic entoderm. The 

 second strip induced predominantly anterior 

 head structures (forebrain, eyes, nose, bal- 

 ancers), the third strip mainly posterior head 

 structures (rhombencephalon, otocysts), while 

 the caudal strip induced regularly spinal 

 cord and, frequently, pronephros and tail, 

 but no brain. However, some structures, such 

 as eye or otocysts, could be induced not only 

 by that part of the mesoderm which under- 

 lies these structures normally, but by adja- 

 cent regions as well. Similar results were 

 obtained by Ter Horst ('48) in explants, and 

 by Okada and Takaya ('42), Okada and 

 Hama ('43, '45) and Hama ('49), who tested 

 the specificity of the cephalocaudal sections 

 of the archenteron roof, before and after their 

 invagination, by implanting them into whole 

 embryos or ectoderm vesicles. These authors 

 confirmed the rather puzzling observation 

 of Mangold ('33b) that the anteriormost 

 part of the archenteron was practically 

 inactive. 



Unfortunately, neither Mangold nor the 

 Japanese authors worked heteroplastically, 

 and they disregarded possible effects of the 

 host levels on the regional specificity of the 

 inductions. However, in view of the consis- 

 tency of their results, the conclusion seems 

 justified that the different levels of the 

 archenteron roof have to a certain degree 

 regionally specific induction capacities. Ob- 

 viously, we are dealing with overlapping 

 and not sharply delimited "induction fields" 

 {Organisationsfeld, Spemann, '21a; Deter- 

 minations f eld, Weiss, '26). 



Indirect evidence for a regional inductive 

 specificity of the archenteron roof may be 

 derived from the experiments of F. E. Leh- 

 mann ('38) in which successive gastrula 

 stages of Triturus taeniatus were exposed to 

 solutions of lithium chloride for short peri- 

 ods. The treatment resulted in localized 

 defects in the anterior or posterior head or 

 trunk mesoderm, respectively, depending on 

 the stage subjected to lithium chloride. 

 Corresponding deficiencies appeared in the 

 overlying neural system. The results were in- 

 terpreted in terms of stage-specific ("phase- 

 specific") and localized susceptibilities of the 

 mesodermal regions to lithium, and a rather 

 strict regional correspondence between meso- 

 dermal inductors and induced neural struc- 



