Selected Invertebrates 



327 



From this brief survey it appears that 

 interaction between blastomeres is of minor 

 importance in the development of nemer- 

 tines, most annelids and mollusks; it appears 

 to be essential to the development of a iev} 

 structures of tunicate larvae, viz., cerebral 

 vesicle, sensory organs, and adhesive papil- 

 lae. By contrast, interaction between blasto- 

 meres appears to be involved in the develop- 

 ment of almost every larval structure of 

 echinoderms. Such interactions have been 

 analyzed most extensively in developing 

 eggs of sea urchins and sand dollars. 



IN ECHINODERM EGGS 



That fragments of sea urchin eggs are 

 capable of considerable regulation (as also 



ingly, the two meridional halves of the sea 

 urchin blastula can regulate into a normal, 

 though small, pluteus larva. Obviously the 

 fate of individual blastomeres or even of 

 groups of blastomeres must become rigidly 

 fixed much later than in the eggs thus far 

 examined; otherwise those in each half blas- 

 tula could not respond to the altered rela- 

 tionships prodviced by such experimental 

 interference by collectively producing a nor- 

 mal individual. 



Existence of Animalizing and Vegetalizing 

 Influences. The sea urchin egg is by no means 

 devoid of organization. Not every part of it 

 can form a whole (as Driesch originally be- 

 lieved); for if the egg is divided into two 

 halves equatorially either before fertilization 

 or at the 8-cell stage by separating the four 



IN WHOLE EGGS BY CHEMICAL TREATMENT 



Fig. 124. Modifications of sea urchin development by operative and chemical procedures {A through 7 

 from Horstadius, '35). A-D, Series of animalizations characteristic of isolated animal halves; increasing 

 degrees of animalization from right to left. A'-D', Series of definitive animalizations arising from transitory 

 types A-D. E-J , Series of vegetalizations characteristic of isolated vegetal halves; increasing degrees of 

 vegetalization from left to right. A'-^, Series of vegetalizations of whole eggs produced by LiCl treatment; 

 increasing degrees of vegetalization from left to right {K, M and A'^ from Herbst, 1896; others from Child, 

 '40). T-Z, Series of animalizations of whole eggs produced by treatment with NaSCN; increasing degrees of 

 animalization from right to left (from Lindahl, '36). 



are fused eggs) has long been known. Each 

 of the first two blastomeres, or each of the 

 first four blastomeres can produce, when 

 isolated, a relatively normal, though small, 

 pluteus larva (Driesch, 1891, 1892; Hor- 

 stadius and Wolsky, '36); even more strik- 



animal cells from the four vegetal cells, the 

 resulting larvae usually differ considerably, 

 not only from each other but also from the 

 normal pluteus (Fig. 124). The most ex- 

 treme larva derived from the animal half is 

 characterized by (1) an apical tuft which is 



