I. NEURULATION 119 



places where the processes of development are most active. 

 Originally, they are to be found in the nucleus of the oocyte; 

 later, they scatter over the egg cortex, especially at the dorsal 

 side. During gastrulation, a considerable accumulation of 

 ribonucleic acid and sulphydryl compounds occurs in the lips of 

 the blastopore, and afterwards in the archenteron roof, in 

 particular in those parts of the cells that face the overlying 

 ectoderm. The inductive powers of dead inductors are pro- 

 portional to their ribonucleic acid contents, and disappear on 

 decomposition of these substances, e.g. by boiling or by enzyme 

 treatment (Brachet, 1945) • All this suggests that these substances 

 play a major role in development, and that they are important 

 for the contact induction of the neural plate as well. It has not 

 yet been ascertained whether or not the latter process is 

 produced by diffusion, from the archenteron roof into the 

 overlying ectoderm, of ribonucleic acid, or of oligonucleotides 

 formed by its hydrolysis. Another view was in recent years 

 put forward by Brachet (1952) ; according to him the micro- 

 somes, which contain the major part of the ribonucleic acid 

 contents of the cell, would pass as such from the cells of the 

 archenteron roof into those of the ectoderm. Finally, mention 

 must be made of experiments by Kuusi (1951) which suggest 

 that different substances play a role in the induction of the 

 rostral and caudal parts of the neural plate (cf. what was said 

 above, p. 115, on the qualitative differences in the inductive 

 powers of the archenteron roof). Ribonucleic acid may be 

 specially important for the induction of the anterior parts of 

 the brain, whereas the evocator for the caudal part of the 

 neural plate may have the properties of a protein. Recent 

 experiments by Nieuwkoop (1952), however, have led to the 

 assumption that the determination of the neural plate takes 

 place in two phases. The first of these, "activation", would be 

 the same in all regions of the neural plate. Possibly it is due 

 to the ribonucleoproteins of the whole archenteron roof. Activ- 

 ation would be followed by a second phase, during which the 

 posterior parts of the central nervous system acquire their 

 regional character. It is conceivable that Kuusi's spinal evocator 

 is the agent causing the second phase. 



