68o PATTERNS AND PROBLEMS OF DEVELOPMENT 



areas, or in change in localization or distribution of areas previously vis- 

 ible." The form of the unfertilized egg in some species — for example, 

 Arenicola (p. 658) — indicates that something more than polar pattern 

 is present. According to Just (191 2), the meridional first cleavage plane 

 in Nereis usually passes through the entrance point of the sperm, and this 

 may be any point of the egg surface. If this is true, a dorsiventral pattern 

 determining direction of cleavage is not present in this egg before fer- 

 tilization. Whether a dorsiventral pattern is present in the egg of Chaelop- 

 terus and some other forms before fertilization seems to be uncertain. "" 



Although there is no general relation between spiral cleavage and 

 lateral asymmetry, since most of the groups with this type of cleavage 

 are not laterally asymmetrical, a relation apparently does exist in the 

 laterally asymmetrical gasteropods. In most gasteropod species the coil- 

 ing is dextral, with occasional sinistrally coiled individuals in some. Cer- 

 tain species, however, are completely sinistral with "situs inversus." In 

 these the cleavage is reversed, that is, cleavages dexiotropic in other 

 forms are here leiotropic, and vice versa.^'' Observation of spiral rays in 

 one aster of the first polar spindle in certain gasteropods led to unsuccess- 

 ful attempts to correlate direction of this spiral with cleavage and direc- 

 tion of coiling.'^ Moreover, a spiral aster appears at the inner pole of the 

 first polar spindle of the annelid Arenicola marina because the spindle 

 forms equatorially and turns through an angle of approximately 90° about 

 this pole (Child, 1898), hut Arenicola is not laterally asymmetrical. Spiral 

 asters are doubtless associated with protoplasmic movement, and it is, 

 of course, possible that direction of movement is correlated with an egg 

 pattern; but evidence of their association with gasteropod asymmetry is 

 lacking. The suggestion by Conklin (1903a, b) that reversal of cleavage 

 and of asymmetry in sinistral gasteropods results from reversal of polarity 

 in eggs of these species after they are freed from ovarian attachment is 

 not supported by evidence, even when position of polar-body formation 

 is altered by centrifuging (Conklin, 191 7). 



The question of the manner of inheritance of sinistrality in gasteropods 

 has received considerable attention but is not yet fully answered. The 

 viviparous genus Partula produces either dextral or sinistral young ir- 



" See, e.g., Conklin, 1902; E. B. Wilson, 1904; F. R. Lillie, 1906; Schleip, 1914; Penners, 

 1922. 



" F. R. Lillie, 1906; Morgan, 1938; Morgan and Tyler, 1938. 



23 Crampton, 1894, Physa; Holmes, 1899, Aitcylus, and 1900, Planorbis. 



'^ Rabl, 1900; see also Schleip, 1929, pp. 111-12. 



