II INTRINSIC MORPHOCHORESIS MECHANISMS 357 



These do evidently play some role in the installation of kinematics or the appearance of a 

 blastoporal lip. Later on, the surface specialization of cells belonging to the various layers and 

 regions (neural plate, young somites, pronephros, endoblast, etc.) is to be considered one 

 aspect of their respective morphogenetic potential. This is the evidence which Townes and 

 Holtfreter (1955, p. 95) fail to consider. In discussing the facts already mentioned, they 

 put the concept we advocate under the heading: "Unsupported hypotheses on morpho- 

 genesis". These authors write: "Dalcq and Pasteels (Dalcq, 1941b) have postulated that 

 invagination of the blastoporal material is caused by an interplay of two hypothetical 

 concentration gradients. It would require much imagination to find support of this 

 hypothesis in the present results". The least that can be said is that such a statement is 

 completely irrelevant to their paper. On the contrary, a simple objective consideration of 

 observations and experimental data obtained from successive stages by all efficient methods, 

 shows a logical and still more satisfactory picture, although it evidently is not yet definitive 

 nor complete. 



III. NEUROGENIC INDUCTION, THE MAIN ASPECT OF 



EXTRINSIC MORPHOCHORESIS 



(with some data on non-neurogenic inductions) 



The classification of processes of induction as processes of extrinsic morphogenesis 

 is perhaps a bit unusual, yet it helps to give them their exact importance in 

 development. Their significance would only be a subsidiary or complementary if 

 the organs, owing their existence to induction were not so essential to Hfe. 



There are several types of induction, and they may be distinguished according 

 to chronology or space. One distinguishes first between primary, secondary, and 

 tertiary inductions, (j) The primary ones occur either inside the middle layer, 

 a process which has already been considered (p. 351) or from this layer on the 

 endoblast (p. 356) or from the middle layer on the ectoblast, the phenomenon 

 which will receive our special attention in this section. At both ends of the gut, 

 the endoblast also apparently acts on the epiblast to form the stomodoeum and 

 the proctodaeum and these relatively late events in morphochoresis can also be 

 classified as primary inductions. (2) Secondary inductions are mainly dependent 

 on the neural tube, mostly on brain parts or their derivates, acting on the epiblast 

 before its determination as epiderm. Sometimes, two or three inductors may act 

 either successively or simidtaneously on one "Anlage", to obtain a definite organ 

 in its full complexity (Holtfreter's coordinated inductor systems, Lehmann's Kombi- 

 native Einheitsleistung) . {3) Tertiary inductions are known, or at least suspected, in 

 the differentiation of the cornea, the teeth, the dorsal fin, and probably many other 

 instances. Some manifestations of rather advanced stages, e.g. the development 

 of the apical cap of limb buds, the experimental induction of a tympanic mem- 

 brane, and the organizing effect of mesenchyme on the salivary gland, are 

 difficult to classify, but are probably relevant to tertiary inductions. 



A certain fascination is attached to the neurogenic induction. When Spemann, 

 in 1 92 1, recognized this entirely new phenomenon, he used the word "Organi- 

 sationszentrum" (see also Spemann and Mangold, 1924) with its broad impli- 

 cations. Further investigations on the "organizer" placed emphasis on the reactor 

 as much as on the inductor. The obstinate search for a biochemical explanation 

 has not yet been successful. Numerous ways of imitating induction by various 

 means have been discovered, but their common factor, if it exists, still escapes the 



Literature p. 483 



