330 



Embryogenesis: Progressive Differentiation 



(Fig. 124F); the skeleton becomes abnormal 

 ''Figs. 124X,W); gut fails to form (Figs. 

 'i24W, V, etc.) and no coelom forms. With 

 increasing animalization an apical tuft also 

 appears at the most vegetal pole (Fig. 

 124F), and with continued animalization 

 spreads in proportion as the animal apical 

 tuft spreads (Fig. 124Z7) until finally the 

 most extreme animalized form (Fig. 124T) 

 is completely covered with apical tuft cilia. 

 Such modifications constitute animalization, 

 since ectodermal differentiations occupy a 

 greater part of the body than normally. 



Hypotheses to Account for Vcgetalization 

 and Animalization by Chemical Agents. 



RUNNSTROM-LINDAHL HYPOTHESIS. RuUnStrom 



('33) suggested that the gradient of animal- 

 izing influences decreasing from the animal 

 pole might be the expression of a graded 

 distribution of animalizing substance, and 

 similarly that the gradient of vegetalizing 

 influences decreasing from the vegetal pole 

 might be the expression of a graded distri- 

 bution of vegetalizing substance. Lindahl 

 ('36) attempted to account for the origin 

 of these hypothetical substances by suggest- 

 ing that there are two specific kinds of 

 metabolism in the egg, carbohydrate metab- 

 olism in the animal half, protein metabolism 

 in the vegetal half, producing animalizing 

 and vegetalizing substances, respectively. 

 Lindahl assumes that lithium treatment 

 blocks carbohydrate metabolism and re- 

 duces the amount of animalizing substance 

 formed, thus reducing the intensity of the 

 animalizing influence and thereby enabling 

 the unmodified vegetalizing influence to turn 

 the development of a larger proportion of 

 the egg than usual in an entodermal direc- 

 tion. And conversely Lindahl suggests that 

 ectodermization is the result of a block to 

 protein metabolism, reducing the amount 

 of vegetalizing substance formed, thereby 

 reducing the intensity of the vegetalizing 

 influence and enabling the unmodified ani- 

 malizing influence to turn the development 

 of a larger proportion of the egg than usual 

 in an ectodermal direction. This hypothesis 

 has stimulated an enormous amount of re- 

 search, far too extensive to be reviewed and 

 evaluated here; fortunately this has been 

 done in recent years by Lindahl ('42), Need- 

 ham ('42), Lehmann ('45), Horstadius and 

 Gustafson ('48), Horstadius ('49) and 

 Brachet ('50). Needham evaluates this hy- 

 pothesis as follows (pp. 494^95): "That 

 there are two centres of morphogenetic in- 

 fluence and a gradient system which reforms 

 after certain experiraental interferences 



must be regarded as established. It is un- 

 deniable also that the actions of these cen- 

 tres may be imitated by a variety of chemical 

 agencies, and it is at least exceedingly prob- 

 able that the two centres are characterised 

 by two different kinds of metabolism. That 

 they produce each a definite morphogenetic 

 substance or substances is likely. But with 

 the statement that the animal pole is asso- 

 ciated with carbohydrate catabolism and 

 the vegetal pole with that of protein, we 

 reach a point where caution is necessary." 

 For more recent attempts to demonstrate dif- 

 ferences in metabolism in animal and vegetal 

 halves, and to relate these differences to dif- 

 ferentiation, see Horstadius ('49), Gustafson 

 and Hjelte ('51) and Gustafson and Lenicque 

 ('52). 



CHiLD-RULON HYPOTHESIS. Instead of a 

 double gradient system. Child ('40) proposes 

 a single gradient system diminishing from 

 the animal towards the vegetal pole. Child 

 interprets the mechanics of vegetalization as 

 follows (p. 29): "If prospective entoderm 

 originates from the lower levels of a primary 

 apico-basal gradient, as dye reduction and 

 differential lethal susceptibility indicate, en- 

 todermization may result from depression or 

 inhibition of prospective ectoderm below 

 a certain 'physiological level'; that is, the 

 specific difference between ectoderm and 

 entoderm may be a secondary result of a 

 nonspecific, primarily quantitative differ- 

 ence. According to this suggestion, ento- 

 dermization occvxrs first in the most basal 

 levels of prospective ectoderm and progresses 

 acropetally with increasing inhibition be- 

 cause lower levels of ectoderm require only 

 relatively slight inhibition, higher levels, 

 more extreme inhibition, to bring them 

 down to the entodermal level." Rulon ('41), 

 accepting Child's concept of a single physio- 

 logical gradient, accounts for ectodermiza- 

 tion by sodium thiocyanate as follows (pp. 

 312-313): "Exposure of vmfertilized eggs to 

 NaSCN in Ca-free sea water apparently re- 

 sults in a general depression or slowing down 

 of physiological processes, the apical or more 

 active region being retarded to a greater 

 extent than the less active region. Such dif- 

 ferential inhibition may be sufficient to de- 

 crease the apical dominance to such a degree 

 that the axiate pattern is partly obliterated. 

 With the stimulation to increased activity, 

 resulting from return to normal sea water 

 and fertilization, the lower levels of the 

 gradient, being wholly or partly physiolog- 

 ically isolated, may recover and develop in 

 the same, or almost the same, way as higher 



