178 CONK LIN. [Vol. XIII. 



divisions. This alternation is essentially the same in both 

 orthoradial and spiral cleavages; in the former case, the 

 axes of the nuclear spindles are alternately meridional and 

 equatorial, in the latter they lie between these positions, being 

 alternately oblique to the right and to the left. 



It is a most remarkable fact that in all known cases of 

 spiral cleavage, with the exception of a few sinistral gastero- 

 pods, the direction of the spirals is invariably the same. The 

 full significance of this fact can only be grasped when one 

 realizes that spiral cleavages are found in animals so far 

 apart as turbellarians, annelids, lamellibranchs, and gastero- 

 pods. 



Selenka ('81) first called attention to the spiral character of 

 the third cleavage by which the first quartette of ectomeres is 

 formed. He also observed that in the formation of the second 

 quartette the spiral was in the opposite direction. Lang ('84) 

 carried the spiral cleavages back to the second division of the 

 tgZ, which he characterized as a " left-wound spiral." As a 

 result of this spiral division, he showed that two of the macro- 

 meres lie at a higher level than the other two, and consequently 

 two polar furrows are formed (see p. 52). These polar furrows 

 always bear a fixed relation to the first two cleavages, because 

 the second cleavage is constantly laeotropic. 



Other investigators have recognized the spiral character of 

 the second cleavage in many other animals, but, so far as I 

 know, no one has suspected that the first cleavage also is a 

 spiral one. This, however, is the case in Crepidula, for imme- 

 diately after the first cleavage is completed, it can be seen that 

 the first division was a dexiotropic one.^ Likewise in all ani- 

 mals in which the second cleavage is constantly laeotropic, it is 

 probable that the first is virtually dexiotropic. The spiral cleav- 

 ages, therefore, probably begin with the first division of the 

 egg, and in almost every case in a dexiotropic direction, the 

 second division is laeotropic, the third dexiotropic, the foiirth 

 laeotropic, etc. In Crepidula these alternating spirals proceed 

 without a break, except slight differences in the time of divi- 

 sion in the different quadrants, from the i- to the 44-cell 



1 See p. 42. 



