Selected Invertebrates 



317 



zontal or oblique section, differ in develop- 

 ment according as they do or do not contain 

 the lower white area. The upper fragment 

 . . . produces a larva similar to the lobeless 

 ones. The lower one . . . may produce a 

 normally formed dwarf trochophore. Frag- 

 ments obtained by vertical section through 

 the lower white area may . . . produce nearly 

 normally formed dwarf trochophores." 



Taken together these experiments on un- 

 cleaved eggs suggest that in the develop- 

 mental history of the egg there is probably 

 no structural organization along an animal- 

 vegetal axis at first, that such a pattern prob- 

 ably arises progressively, and that it may 

 be realized earlier in some uncleaved eggs 

 (sea urchin, mollusk) than in others (nemer- 

 tine, tunicate). This is the same conclusion 

 that was reached concerning the establish- 

 ment of visible differences along the axis of 

 polarity. 



ARE EXTRINSIC OR INTRINSIC FACTORS 



RESPONSIBLE FOR ESTABLISHMENT 



OF AN AXIS OF POLARITY? 



As a general rule the attached end of the 

 egg in the ovary becomes the vegetal pole, 

 the free end the animal pole. There are those 

 who believe that this relationship is a causal 

 one, such that the different environments 

 at the two ends of the egg cause those ends 

 to develop differently. If this were true, the 

 basic organization of the egg along the 

 animal-vegetal axis would be imposed upon 

 it from the outside. Child ('41) has empha- 

 sized the importance of extrinsic factors in 

 the establishment of polarity in animal eggs. 

 Although such a causal relationship may 

 actually exist, it has not been proved experi- 

 mentally for eggs of invertebrate animals, 

 and such proof would be difficult to obtain. It 

 is equally difficult to prove that such a causal 

 relationship does not exist. 



Be this as it may, one important feature 

 of egg organization, at least in snails, ap- 

 pears to be controlled by intrinsic factors, i.e., 

 by the genotype of the unreduced egg. The 

 resulting egg organization in some way 

 controls (1) the position of the cleavage 

 spindles for the second cleavage, whether 

 dexiotropic or laeotropic; (2) the position of 

 the primary mesoderm cell, 4d, whether to 

 the left or the right of the first cleavage 

 plane; (3) the direction of coiling of the 

 visceral mass and shell, whether clockwise 

 or counterclockwise. Sturtevant ('23) has 

 suggested that one pair of genes is involved, 

 a dominant dextral factor and a recessive 



sinistral factor. In terms of the usual Mende- 

 lian inheritance a cross between two hetero- 

 zygous parents should give 3 dextral: 1 

 sinistral offspring; actually all are dextral. 

 A cross between a homozygous recessive 

 female from the above cross and a homozy- 

 gous dominant male should give all dextral 

 offspring; actually all are sinistral. Thus the 

 genotypic make-up of the egg seems to have 

 exerted a permanent effect upon the egg 

 organization prior to onset of the maturation 

 divisions. Even though Sturtevant's inter- 

 pretation may not accotmt for the results of 

 all crosses (consult Morgan, '27), this does 

 not detract from the generalization that in- 

 trinsic, as well as extrinsic, factors play im- 

 portant roles in the establishment of egg 

 organization. 



STRATIFICATION OF EGG CONSTITUENTS 



ALONG THE AXIS OF POLARITY IS THE 



RESULT, NOT THE CAUSE, OF POLAR 



ORGANIZATION 



It can be demonstrated experimentally by 

 centrifugation that the specific distribution 

 assumed by the various visible constituents 

 of the egg does not create the basic polar 

 organization which underlies regional dif- 

 ferentiation, but is a consequence of that 

 polar organization. The latter basic organiza- 

 tion remains unchanged if a new axis of 

 stratification of visible constituents is im- 

 posed by centrifugation (although the posi- 

 tion of maturation spindles and cleavage 

 planes may be modified to conform to the 

 axis of centrifugation). Thus in sea urchin 

 eggs the axis of stratification imposed by 

 centrifugation was found to bear every pos- 

 sible relationship to the axis of differentiation 

 (Morgan, '27), and a similar situation 

 characterized the eggs of the annelids 

 Chaetopterus and Nereis (Lillie, '09), of 

 such mollusks as Cumingia (Morgan, '27), 

 Physa and Lymnaea (Conklin, '10), and of 

 the tunicate Styela (Conklin, '31), etc.* It 

 therefore appeared that polar organization 

 of the egg was a property of the ground sub- 

 stance, by which Lillie ('06, p. 156) meant 

 the "fkiid that contains and suspends all the 

 granules and droplets." According to Lillie 

 ('09), polarity must depend on some definite 

 architecture of the ground substance, and 

 movements of visible constituents produced 

 by the centrifuge cannot be mass movements 

 of entire protoplasmic areas, but only gran- 

 ule movements through the ground sub- 



* Raven and Bretschneider ('42) disagree with 

 this conclusion. 



