ALBERT

Notes: Summary The question about the nature of the effect of lithium on early sea urchin development is reexamined. Essential features of the morphogenetic changes of lithium treated embryos are followed with continuous comparisons with control embryos. The key to the lithium effect is the greater cytoplasmic susceptibility in the animal polar region as compared with the vegetal one. This is diagrammed in Fig. 23a-c on the basis of the double gradient concept. There is a decline of the animal gradient with increasing lithium concentration. As a consequence the level of differentiation of the terminal region becomes more and more vegetal, seean/veg-values in Fig. 23. The region suppressed by a prolonged exposure to lithium, e.g. 9–16 hours, cannot be restored. Nevertheless, there are data from previous research supporting the view that the primary effects of lithium are reversible within certain limits. However, when the normal balance is disturbed by decline of the animal gradient and particularly by suppression of its higher levels, there is a compensatory enhancement of the vegetal gradient system which stabilizes the suppression. As a consequence of the suppression of the higher animal levels, a certain accumulation of cells in an anterior direction has taken place in the blastula stage. The degree of accumulation reflects the degree of vegetalization. Later there is to varying extent a backflow of cells in the vegetal direction. It was shown how a great part of the blastula wall may have the aspect of an attachment zone (Fig. 7). The primary mesenchyme cells attach themselves only to a certain level of the ectoderm in which the relativean/veg-value is around 0.7 according to the conventions behind Fig. 23. Sections of Carnoy fixed embryos were exposed to trypsin. It proved that the external cytoplasm of blastodermic cells in lithium treated embryos was more strongly attacked than the internal one. The latter showed a strong resistance to tryptic action. On the other hand, in the control embryos the inner part of blastodermic cells was completely digested with exception of the vegetal region including the attachment zone. The trypsin resistant structure may be preformed and may be responsible for the higher rigidity of the cytoplasm in lithium treated embryos (section IIIf). It is proposed that in the period of lithium susceptibility, the colloidal state is most affected in the animal region, thereby creating a block to the diffusion of the animalizing substances which results in the shifts diagrammed in Fig. 23.