252 



Embryogenesis: Progressive Differentiation 



brains, sense organs, somites, pronephros, and 

 others. The different structures occvirred at 

 approximately the same sites as the cor- 

 responding host structures, their frequencies 

 diminishing with the distance from these 

 sites. This indicates that the dorsolateral 

 mesoderm, like the more median archenteron 

 roof, consists of a cephalocaudal series of 

 specific, yet overlapping, induction fields. 

 Some of the differentiations, such as bal- 

 ancers, gills, or ear vesicles, revealed the 

 presence of induction fields outside the med- 

 ullary plate which operate in normal devel- 

 opment (see p. 255). However, the occur- 

 rence of supernumerary brains or tails in a 

 lateral position seems to show that the neural 

 plate inductors actually extend farther into 

 the lateral mesoderm, beyond the boundary 

 of the neural plate. It is probable that these 

 peripheral induction capacities are normally 

 not expressed because the ectoderm of the 

 neurula- — in contrast to the grafted gastrula 

 ectoderm — is no longer competent to respond 

 to them (p. 257). 



Corresponding experiments on still older 

 hosts have shown that these induction fields 

 remain potentially active far beyond the 

 stage at which they are normally engaged in 

 organ determination. But a significant dif- 

 ference exists between these normally in- 

 active indvictors and the early organizer: 

 Whereas the latter tends to assimilate the 

 induced mesodermal tissues into its own field, 

 the variovis tissues induced by the aged in- 

 ductors are not incorporated but tend to es- 

 tablish axial systems of their own. For 

 discussion of this problem see p. 279. 



ORGANIZATION AND EARLY 



DIFFERENTIATION OF THE MEDULLARY 



PLATE 



After it had become evident that the dif- 

 ferent regions of the archenteron roof are 

 instrumental in blocking out the major sub- 

 divisions of the medullary plate, the question 

 arose: To what extent is the subjacent meso- 

 derm responsible for the more detailed pat- 

 terning of the central nervous system? Does 

 it operate merely as a short-term "activator" 

 or does it exert an influence over a longer 

 period participating also in the determina- 

 tion of the tissue organization of the induced 

 areas? 



An examination of this question revealed 

 that the capacity for neural differentiation 

 and organization does not arise abruptly but 

 develops progressively during the protracted 

 period of gastrulation. This was demonstrated 



by transplantations and explantations of cir- 

 cumscribed regions of the prospective or 

 visible medullary plate of Triturus alpestris 

 (Mangold, '29a; Mangold and von Woell- 

 warth, '50). A merely epidermal differentia- 

 tion occurred when the material was re- 

 moved before it had contact with invagi- 

 nated mesoderm. When taken from middle or 

 late gastrulae which already possess an arch- 

 enteron roof, the pieces developed partly into 

 neural structures, but the percentage of iden- 

 tifiable brain parts and eyes was low. From 

 early neurula stages on, the majority of the 

 cases formed typical brain parts and single 

 or synophthalmic eyes. Gallera ('47, '48) 

 and Damas ('47) obtained similar results 

 with Pleurodeles. However, they stressed 

 the point that the grafts from the younger 

 stages differentiate into pigment cells rather 

 than into neural tissue and they assumed that 

 this stage-linked differentiation reflects quan- 

 titative dosage effects of the inductive sub- 

 stratum. 



After the medullary plate has become 

 visible, its different levels are capahle of 

 differentiating into typical fore- or hindbrain, 

 eyes, or spinal cord, respectively, when iso- 

 lated from the underlying mesoderm (Man- 

 gold, '33b; Nakamura, '38; Ter Horst, '48; 

 and others). However, these regions represent 

 but indistinctly outlined morphogenetic 

 fields whose parts are not yet rigidly de- 

 termined. A variety of experiments has 

 shown that this applies to urodeles as well 

 as to anurans (for references see Mangold, 

 '31a; Adelmann, '36). Extirpations of large 

 parts of the anterior medullary plate, or a 

 rotation through 90 or 180 degrees of the 

 median portion of this region, without the 

 underlying mesoderm (Alderman, '35) failed 

 to produce marked abnormalities in brain 

 and eye development. Small pieces of the 

 prospective spinal cord region implanted in 

 the brain region were assimilated by the 

 latter (Umanski, '35). Hence, the anterior 

 part of the medullary plate does not repre- 

 sent a mosaic pattern of cephalic primordia 

 but a general and labile eye-forebrain field. 

 This field seems to possess a mediolateral 

 gradient because heterotopic transplants of 

 its median strip formed an eye six times as 

 frequently as did grafts of lateral strips 

 (Adelmann, '30). Its anteroposterior polarity 

 is fixed as early as the neural groove stage, 

 as was shown by rotation experiments of the 

 entire anterior prospective medullarv plate 

 or of its lateral half TRoach, '45V (In the 

 above-mentioned rotation experiments of 

 Alderman, the transplants were much smaller 



