Selected Invertebrates 



329 



can still be induced from cells of the animal 

 half, and the necessary shiftings of the posi- 

 tion of stomodaeum and ciliated band can 

 still take place such that an almost normal 

 pluteus results. Thus the animal half is 

 competent to respond to vegetalizing influ- 

 ences of the micromeres at a stage later than 

 it is capable of self-differentiation. This must 

 mean that determination is progressive, that 

 when it has advanced far enough to enable 

 an isolated region to self-differentiate, it is 

 still only a labile determination and can 

 be changed if brought under appropriate in- 

 fluences.* 



Do Animal Halves and Vegetal Halves Lose 

 Their Capacity to Exert Animalizing and 

 Vegetalizing Influences in Advanced Stages? 

 Horstadius ('50) demonstrated that an ani- 

 mal half from an advanced stage (blastula 

 before formation of primary mesenchyme) 

 can still exert animalizing influences on 

 much younger vegetal halves, and similarly 

 that a vegetal half of a beginning gastrula 

 can still exert vegetalizing inflviences on a 

 younger animal half sufficient to inhibit 

 extension of the apical tuft and evoke for- 

 mation of a ciliated band and stomodaeum. 



Graded Intensity of Animalizing and Vege- 

 talizing Influences. Transplantation experi- 

 ments (Horstadius, '39) sviggested that ani- 

 malizing and vegetalizing inflviences were 

 not distributed uniformly throughout the 

 animal and vegetal halves respectively, but 

 existed as overlapping gradients with po- 

 lar concentrations. Animalizing influences 

 seemed strongest in an-^^ less strong in a^o, 

 less still in veg^ and least in veg^ ; vegetaliz- 

 ing influences seemed to be strongest in the 

 micromeres, less strong in vegc,-^ weaker still 

 in vegi and weakest in the animal hemi- 

 sphere. Normal development appeared to de- 

 pend upon a proper balance between ani- 

 malizing and vefi:etalizing inflviences, rather 

 than upon their absolute intensities. The 

 evidence for this has been reviewed by 

 Horstadius ('39, '49). 



Chemical Modifications of Development. 

 Vesetalization (Entodrrmization) of Entire 

 Eggs. If entire sea urchin egp-s are fertilized 

 in normal sea water and are then transferred 



* It should be noted that animal halves retain 

 their competence only if they remain in contact 

 with the veg^etal halves until iust before micromeres 

 are added: if animal halves are isolated 4 hours 

 after fertilization and the isolated animal halves 

 remain in isolation until 16 hours nostfertilization. 

 at which time micromeres are added, the animal 

 halves are no longer competent to respond to the 

 vegetalizing influences. They form no stomodaeum. 

 no ciliated band, no restricted apical tuft and no gut. 



to sea water containing lithium chloride, 

 many of the resulting larvae resemble in a 

 striking way those derived from isolated 

 vegetal halves (Herbst, 1892, 1893, 1896; 

 Runnstrom, '28a; Child, '40). This is very 

 evident upon comparison of Figure 124K 

 with 124F, 124L with 124G, 124M with 

 124//, 124A^ with 1247,7, etc. Treatment 

 with lithium apparently shifted the bound- 

 ary between ectodermal and entodermal de- 

 velopment above the equator (i.e., prospec- 

 tive ectoderm was converted to entoderm) 

 such that an excessively large gut devel- 

 oped.f That this is the correct interpreta- 

 tion is indicated by two types of informa- 

 tion. It is known that primary mesenchyme 

 cells aggregate into two masses at a level 

 along the animal-vegetal axis determined by 

 the ectoderm (von Ubisch, '39b); a skeletal 

 spicule forms within each such aggregation. 

 Following lithium treatment these micro- 

 mere derivatives aggregate much closer to 

 the animal pole, suggesting that the lithium 

 treatment has restricted ectodermal differen- 

 tiation to a level closer to the animal pole. 

 Moreover, the dark-field ring mentioned 

 earlier as associated with the cortex of pro- 

 spective entodermal cells likewise extends 

 further towards the animal pole in lithium- 

 treated eggs (Runnstrom, '28a, b). Lithium 

 can also modify development of isolated ani- 

 mal halves in the same direction as micro- 

 meres do (von Ubisch, '29) ; lithium-treated 

 animal halves can develop into pluteus-like 

 larvae iust as they can when micromeres 

 are added. 



Animalization (Ectodermization) of Entire 

 Eggs. If entire sea urchin eggs are treated 

 before fertilization with calcium-free sea 

 water to which isotonic sodium thiocyanate 

 has been added (Lindahl, '36), and are 

 then returned to sea water, are fertilized and 

 allowed to develop, many of the resulting 

 larvae resemble in a striking way those de- 

 rived from isolated animal halves. With in- 

 creasing degrees of animalization modifica- 

 tions occvir in the following sequence: the 

 gut fails to connect with the stomodaevim 



f Only such development is true vegetalization. 

 According to Lindahl ('42), only a few treatments 

 result in true vegetalization, and of these lithium 

 is definitely most effective. Vegetalization and exo- 

 gastrulation are not sjmonymous; there are many 

 chemical and physical agents which produce simple 

 exoeastrulation of the type illustrated in Figure 

 12471 without increasing the ratio of entoderm/ 

 ectoderm (see Child, '41. footnote, p. 222"). Extreme 

 reduction of ectoderm and enlargement of entoderm 

 occurs when lithium-treated embryos are returned 

 to sea water (Figs. 1240-^). 



