356 GERMINAL ORGANIZATION INDUCTION PHENOMENA 4 



morphogenetic factors". From fertilization to incipient gastrulation, the whole egg cannot, 

 in my opinion, represent more than one field. In passing, let us note the danger of using 

 such expressions as "self-organizing systems of morphogenetic factors". We have as much 

 reason to protest against such illusory expressions as our predecessors of fifty years ago had 

 against entelechy. 



To item c § j. This statement is more satisfactory, although it oversimplifies the ex- 

 planation of regulation. The problem is, why blastomeres of certain eggs, and not of others, 

 are able to retrace the earlier steps of development. This probably depends on the gradual 

 distribution of the various kinds of organelles. Regulation is a potential ability of all eggs; 

 it is only the period during which they retain this capacity which varies. Let us mention 

 here that induction has a special significance. As Medawar (1954) rightly hinted, induction 

 adds to development an additional degree of freedom. As such, induction is an endowment 

 of eggs belonging to the highly evolved classes, i.e. vertebrates, echinoderms, mollusks (see 

 Raven, 1958), insects. 



To item d. If morphogenetic factors have been mixed, and if, thereby, a complex 

 situation of overlapping fields has arisen, it becomes necessary to postulate a segregation 

 of these factors. But this return to a workable simplicity is again illusory. Development 

 proceeds really from apparent simplicity towards evident complexity. Gastrulation, if we 

 think of vertebrates, is a period of complication, not one of simplifying segregation of 

 imaginary potencies. The period when the egg was still one unitary field comes to an end 

 at this time, because, as a consequence of activities inherent to the primary field, kinematics 

 has started. From now on at least two fields exist, one pervading the invaginated material, 

 derived from the primary field, and one resulting from the induction exerted by the inner 

 material upon the external layer. It is likely that very soon a third field will express the 

 induction of the mesoblast on the endoblast; the results of T, S. Okada (1953, 1956, 1957) 

 afford an experimental basis for this assertion. 



Whether or not the prosencephalic factors as such reside in the blastoporal lip will be 

 discussed later. The possibility that some special properties proceeding from the dorsal 

 half of the normal egg exist in the neurectoblast is not excluded, although these properties 

 are not indispensable to the formation of a normal nervous system. However, it is certainly 

 unnecessary to imagine that these properties originate in the grey crescent and have 

 climbed towards the pole through the micromeres or the ectoblast. If there does exist 

 some very slight influence of the neurectoblast, it simply owes its existence to the primitive 

 unitary field. 



And now, says Ten Gate, "the embryonic fields come into the fore-ground . . .", and, 

 the advantage of the system he has adopted is that he immediately attains an explanation 

 of the biochemical specificity of organs. Behind his "segregation of morphogenetic factors" 

 is concealed the idea of representative particles, which have only to multiply in order to 

 yield specificity. But this is, in fact, a purely hypothetical representation, and indeed, too easy 

 a solution. How the establishment of secondary fields could be more preferably explained 

 will however be more easily examined after a study of neurogenic induction (p. 472). 



To item e § /. is only a statement of facts, and avoids the difficult problem of explaining 

 the so-called determination. We have already met this same enigma (p. 320), with only a 

 relative success. 



To item e gs-j. This passage refers to the new data concerning cell adhesiveness and 

 reciprocal affinities, which have been extended still further in the last three years, and are 

 among the most striking of Holtfreter's numerous investigations. The statement that a 

 "simple diffusion of factors" can explain the "spreading" is indeed questionable. It is 

 more essential to appreciate the real importance of cell affinities in the general picture of 

 early development, without committing the frequent error of seeing in the latest discovery, 

 whether it be regulation, germinal localizations, genes, organizers, or affinities, the key 

 to all the enigmas of ontogenesis. Surface properties of the blastomeres and of their deri- 

 vatives do not exist a priori; they are acquired progressively, and are thus relevant to 

 epigenesis. As far as the eggs of the anamniots are concerned, especially those in these in- 

 vestigations, the trend of ideas which considers reactions between endoplasmic components 

 and the cortex fits in well with the concept of gradual transformation of surface properties. 



