414 



GERMINAL ORGANIZATION 



INDUCTION PHENOMENA 



This is, however, only one among the many interesting effects of this simple treatment. 

 They are obtained (Pasteels, 1953, 1954) with a rather moderate centrifugation (460 g 

 for I to 10 min.). The typical reaction of an autonomous secondary embryo which develops 

 on the ventral side of the "host" is best encountered in Rana temporaria after centrifugation 

 of the blastula (age of 19 h. at 20 C). The effect practically fails with "middle" blastulae 

 (15 h.) and disappears as soon as the blastopore becomes crescent shaped. The phenomenon 

 is indeed complex. It has been studied in 3 Anuran and 4 Urodele species; it is present in 

 all of them except the newt, although there are considerable differences in susceptibility. 

 It also varies according to the stage treated and the duration of application. Even the 

 maximal frequency of the various organs produced, chorda, somites, pronephros, neural 

 axis is not attained simultaneously, but varies with slightly different treatments. There 

 are also conditions in which the modification of the ectoblast expresses itself only by 

 inconspicuous anomalies. In certain species, like the axolotl, the anomaly concerns only 

 the structure of the brain which, moreover, may be complicated by supplementary parts, 

 of either acrencephalic or chordencephalic nature. Chordalizations of the neuroepithelium 

 often take place, even inside acrencephalic organs, e.g. the optic vesicle (Fig. 74). 



When a definite secondary system is evoked, it often shows connections with the primary 

 one; in many cases, action at a distance has been observed, which strongly suggests a 

 diffusion of active "substances" impregnating or imbueing the intermediate region. The 

 transformation of the prechordal anlage into notochord could even be demonstrated in 

 two cases in which, correlatively, the brain has lost any acrencephalic character. These 

 observations prove that features normally characteristic of the notomerit may easily be 

 evoked from the prechordal region, which means either that their factors are potentially 

 present, or that a reconversion of the acrogenetic factors is easily obtainable. 



There are three important aspects of this experiment. First, when an autonomous 

 secondary system is obtained, it is cephalized in the same direction as the normal system 

 (causal relation, as demonstrated by transplantations). Secondly, the additional brain 

 never shows any of the acrencephalic structures ; the secondary system is strictly notomer- 

 itic. Frequently, the predominance of this tendency makes that many cells of the spinal 

 cord become vacuolized, as if of notochordal nature (Fig. 75). Clearly the conditions 

 causing neuralization and chordalization are intimately related. 



Fig. 75. Abnormal structures caused by centrifugation of a frog blastula. The secondary 



system here is to the right of the rhombencephalon. It consists of an enlarged notochord 



and a brain part in which numerous cells are "chordalized". From Pasteels, 1953. 



