Invertebrates 



665 



polarity is relatively fixed and may be com- 

 pared in this respect with the polarity of a 

 sea urchin egg. On the other hand the stems 

 of coelenterates and the stolons of ascidians 

 exhibit a very labile polarity as regards re- 

 generation. A fragment isolated from the 

 stem of Tubularia may form a hydranth at 

 the apical end and a stolon at the basal end 

 and thus an apicobasal polarity is indicated. 

 However, other fragments may regenerate a 

 large hydranth at the apical end and a small 

 hydranth at the basal end. Thus the polarity 

 of isolated fragments is not very strong. The 

 stolons of ascidians show a very weak polar- 

 ity and zooids usually differentiate at both 

 ends of a fragment. The weak initial polarity 

 of ascidians and coelenterates has its coun- 

 terpart in the egg of Fucus, in which polarity 

 may be determined by a number of physical 

 and chemical factors. 



The differences in the kind of polarity, 

 exhibited by fragments of Dugesia on the 

 one hand, and by Tubularia and ascidians 

 on the other, may be simply a matter of 

 intensity since short fragments of Dugesia 

 may develop a head at both the anterior 

 and posterior cut surfaces. Again using the 

 sea vu-chin egg for comparison with Dugesia 

 regenerates, recall that treatment of the 

 former with potassium thiocyanate and 

 lithium chloride results in bipolar forms 

 (Lindahl, '36). 



The coelenterate stem and ascidian stolon 

 possess weak polarities which may be modi- 

 fied by experiment. Chemical and physical 

 gradients applied along the length of the 

 regenerates impress a strong polarity on them 

 and unipolar forms result (Lund, '25; Miller, 

 '39). We thus come to regard regenerates as 

 possessing polarities similar to those of eggs 

 and subject to alteration by experimental 

 procedures (Child, '42). 



The polarity of the regenerate is derived 

 from the polarity of the whole organism. 

 The fact that as many as sixteen fragments 

 of a flatworm will regenerate with antero- 

 posterior polarity makes very unlikely the 

 localization of any morphogenetic substances 

 as accounting for polarity. Therefore the 

 polarity of forms such as Dugesia appears 

 to be maintained chiefly by means of quan- 

 titative differences between anterior cells and 

 posterior cells. On the other hand, polarity 

 in some annelids has a qualitative aspect as 

 well as a quantitative one, since heads do 

 not regenerate from posterior segments. Even 

 in the case of annelids the differences under- 

 lying polarity may be considered as purely 

 quantitative if the additional assumption is 



made that below a certain quantitative 

 threshold regeneration of a head does not 

 take place. If, for example, we assume that 

 head regeneration requires more energy than 

 tail regeneration, then it may be that the 

 energy sources in the posterior segments 

 are too low for head regeneration but still 

 high enough for tail regeneration. 



The simplest hypothesis as regards polar- 

 ity assumes the existence of a single gradient 

 possessing a high intensity at the anterior 

 region and a low intensity in posterior re- 

 gions (Child, '42). This gradient, which is 

 expressed as graded rates of regeneration in 

 Tubularia and as a graded head frequency 

 in Dugesia, is detectable by a variety of 

 methods (Child, '42). However, the physico- 

 chemical natvire of the gradient escapes us 

 as yet. As a physicochemical basis. Child 

 has suggested graded rates of oxidations 

 which would result in graded rates of energy 

 production; Dalcq and Pasteels suggest a 

 yolk gradient in the amphibian egg; Lin- 

 dahl's studies suggest for the sea urchin 

 egg a gradient in carbohydrate metabolism 

 for the animal gradient and a vegetal 

 gradient which may be concerned with pro- 

 tein catabolism. Sulfhydryl gradients have 

 been demonstrated in a variety of forms. 

 New gradients of ribonucleoproteins have 

 been described in the amphibian egg and 

 thus these substances become another possi- 

 ble mediator of the gradients associated with 

 polarity (Brachet, '50). 



A somewhat more complicated gradient 

 theory involving two factors in graded con- 

 centration fits some of the facts of regenera- 

 tion. Two-gradient hypotheses have been 

 formulated in some detail by Ruimstrom 

 for the sea urchin and by Dalcq and 

 Pasteels for the amphibian egg. As regards 

 regeneration, Liebmann ('43) has assumed 

 a two-gradient mechanism for the facts ob- 

 served in regeneration of an annelid. In 

 this instance polarity is assigned to two 

 kinds of cells present in different nxmibers 

 in anterior as compared with posterior seg- 

 ments. These two kinds of cells determine 

 head regeneration and tail regeneration, re- 

 spectively. Since these cells, "head" eleocytes 

 and "tail" eleocytes, respectively, must mi- 

 grate polarly it is clear that there must be 

 a basic polarity in addition to the eleocytes. 



The one-gradient theory of polarity carries 

 with it the corollary of dominance by which 

 high regions inhibit low regions in the 

 gradient. The nature of this inhibition will 

 be considered later. The two-gradient theory 

 involves the concept of antagonism and bal- 



