EXPERIMENTAL MODIFICATIONS 



413 



interesting feature is one which has so far remained completely enigmatical: the 

 formation of supplementary neural material on the dorsal side of the spinal cord 

 or even of the brain. Inspired by Takaya's results (p. 405), I have re-examined 

 the most striking example of this anomaly (Fig. 73). The zone of contact between 

 the dorsal part of the hind-brain and the epiblast is definitely atypical. More 

 than the normal amount of mesenchyme is present {cf. p. 407, note i), and it 

 vaguely surrounds large oblong ampullae, which were certainly full of fluid in 

 vivo; they are interposed between the neural tissue and the covering epiblast. 

 The cause of the inward proliferation of the neuroepithelium could well lie there. 

 Centrifugation^ is a classical procedure for changing the relations of the cell- 

 components without interrupting the course of the vital activities. In amphibian 

 eggs, it has been used on a large scale (literature in Schleip, 1928) and several 

 investigations of the prewar period analysed the interesting effects exerted on 

 morphochoresis in eggs centrifuged during the earliest stages (literature in 

 Pasteels, 1940). It must be remembered here that this technique, especially when 

 applied to undivided frog eggs, hampers, according to the action intensity, the 

 appearance (or at least the utilization) of the acrencephalic factors of induction, 

 and thereby provokes hypomorphosis of the notomerit strikingly parallel to the 

 effects of lithium (see p. 423). 



Fig. 74. An effect of centrifugation of frog gastrulae on the differentiation of the primary 

 acromerit. The right optic vesicle is largely transformed into notochordal cells. One small 

 otocyst has even been formed. This is a real interpenetration between aero- and deutomerit. 



From Pasteels, 1953. 



These results of Pasteels speak in favor of a primary unity in the inducing factors. By 

 exploring the successive stages, a remarkable manifestation was discovered: the formation 

 of a secondary axial system originating from the collapsed part of the blastocoele roof. 



' The data briefly summarized in this section could be advantageously compared with 

 the results obtained in invertebrate eggs, especially in Limnaea (Raven and Beenakkers, 

 19555 see also Raven, 1958). 



Literature p. 483 



