Influence of Implanted Micromeres on Reduction Gradients and Mitochondrial Distribution in developing Sea-urchin Eggs

C H I L D (1936) has described reduction gradients in different stages of developing sea-urchin and starfish larvae. His results concerning the gradients in seaurchin mesenchyme blastulae and early gastrulae have been confirmed with Janus green. There exists a stronger vegetal (acropetal) reduction gradient and a weaker animal one, appearing later, in the normal whole larva. Isolated vegetal halves show a similar vegetal reduction gradient, but in most cases no animal gradient. In isolated animal halves the reduction starts in the thick plate of the enlarged apical tuft and no vegetal gradient appears. Micromeres implanted laterally in the animal part of a whole egg induce there a reduction centre. Four micromeres implanted into an isolated animal half induce a new vegetal reduction gradient and restrict the animal gradient, so that the reduction gradient system now resembles that of a normal egg. A comparison of isolated animal and vegetal halves shows that the reduction seems to start at the same time and to proceed simultaneously in both kinds of halves. We can, therefore, state that the metabolism in isolated animal halves is not identical with that of the same material when forming the animal part of a normal larva. A comparison of the reduction gradients in these different cases shows a remarkable conformity with the animal and vegetal morphogenetic gradients known from the analysis by fragmentation and transplantation experiments. (These results have been published in full, Horstadius, 1952.) Lindahl (1936) has suggested that the animal principle in the sea-urchin egg is connected with carbohydrate metabolism, whereas the vegetal one in later stages might be characterized by a breakdown of proteins. Gustafson (1952) has contributed evidence that mitochondria contain a series of enzymes involved in protein synthesis. The animal metabolic type favours the development of mitochondria whereas the vegetal produces inhibitors for mitochondrial development. In a normal late blastula or early gastrula the mitochondria thus decrease in number from the animal towards the vegetal pole. In corresponding