470 GERMINAL ORGANIZATION INDUCTION PHENOMENA 4 



tail without transitional structures, or a deutomerit appearing at the end of a 

 trunk. In spite of some fascinating accomplishments, the experimenter remains 

 subordinated to law inherent to vertebrate organization. 



The part played by induction in the construction of the nervous system is an 

 accomplishment which legitimately deserves our due attention. In the normal 

 course of events, the neural plate appears as a progressive cephalo-caudal trans- 

 formation involving a characteristically delimited field, at first uniform at any 

 transverse level. The process looks continuous, but the shape of the young plate, 

 with its gradual restriction of breadth behind the cerebral region and the rather 

 uniform width of the spinal part, suggests some duality in the subjacent inductor. 



In this first step of neuralization, we may speak with Waddington of an 

 evocation of the neuroepithelium. All cell functions are apparently reinforced, and 

 ribonucleoproteins and alkaline monophosphoesterase are produced. These 

 cvtochemical data suggest many other changes in metabolites and enzymatic 

 equipment. The cells grow either by utilization of their yolk or thanks to imbibi- 

 tion by a nutrient flow, processes which imply further syntheses of proteins. The 

 DNA metabolism is soon involved, cell proliferation takes place, and a columnar 

 arrangement of the epithelium appears, probably due to the orientation of macro- 

 molecules inside the cytoplasm and to special surface affinities which draw the 

 elongated cells toward the medio-sagittal plane with a convergence-extension 

 movement. It will be useful to state here that this initial set of events is fundament- 

 ally the same at the beginning of later, less extensive inductions. 



Evocation of the neural plate leads to a mass transformation, in close correlation 

 with morphochoresis of the inner materials. These independent events correspond 

 to Waddington's concept of individuation. The concentration of cell toward the me- 

 dio-sagittal plane combines with an active elongation of the whole plate, which 

 also begins to fold, more rapidly around the head-trunk limit. The cerebral region 

 takes form as a whole and its closure proceeds slowly toward the neuropore, while 

 the presumptive medulla already shows more individuation by forming the power- 

 ful cephalic neural crests. While the cerebral vesicle is still closing anteriorly, the 

 anterior half of its floor partly bulges into optic vesicles, the rest becoming the di- 

 and telencephalon. During the relatively late stages of this process, more of the 

 neuroepithelial cells begin to throw off the thin elongating extensions which 

 characterize them as neuroblasts. 



Thus, cytodifferentiation follows a prolonged phase of participation in mor- 

 phochoresis, during which the neural axis acquires its typical shape. However, 

 cytodifferentiation does not necessitate the previous completion of these more 

 general individuation processes. Induction in vitro (Niu) clearly shows that after 

 due time, neuroblasts, ectomesenchyme, and also myoblasts (the latter by para- 

 genesis) can be obtained directly without global morphochoresis. Thus, induction 

 endows the cells with the complements necessary and sufficient for their cyto- 

 difTerentiation. This statement may be considered of general value. Of course, the 

 question whether all kinds of cells need to be induced in order to diflerentiate 

 remains unanswered. One could be tempted to decide that chordal, somitic, and 

 endoblastic cells differentiate by their own resources. However, the fact that any 

 explant from the corresponding areas, no matter how tiny it may be, develops 



