CHAPTER XIV 

 CLEAVAGE AND DEVELOPMENTAL PATTERN 



IF NUCLEI are primarily alike as regards hereditary potentialities, 

 the physiological basis of embryonic developmental patterns must 

 be sought in the egg cytoplasm; and the question whether, or to what 

 extent, the widely different cleavage patterns of different animals are re- 

 lated to developmental pattern arises. Some cleavage patterns — for ex- 

 ample, those of annelids, mollusks (except cephalopods), ascidians, and 

 some other forms — appear to be intimately related to developmental pat- 

 tern; they are highly determinate, that is, certain cells give rise to certain 

 organs or parts by a definite cell lineage, and at least some of the cells, 

 when isolated, are capable of continuing development for a time with 

 little or no change. For this reason earlier stages of these forms have 

 often been regarded as "mosaics" of independently developing parts. At 

 the other extreme are completely indeterminate cleavage patterns with- 

 out any definite relation to developmental pattern, as in polyembryonic 

 bryozoa, in insects and probably most other arthropods, and in mero- 

 blastic vertebrate eggs. Between these extremes are various degrees of 

 determinate character. In other, less speciahzed forms of development, 

 budding, fission, and reconstitution of multicellular forms the single cell 

 has no definite relation to developmental pattern. This suggests that the 

 more highly determinate types of cleavage may be expressions of a certain 

 degree of determination or differentiation of regions of the undivided 

 egg. This chapter is largely concerned with the more highly determinate 

 cleavage patterns and with questions regarding their mosaic character 

 and relation to pattern of the organism. 



SPIRAL CLEAVAGE PATTERN 



As far as mitotic spindles and cleavage planes are concerned, so-called 

 ''spiral cleavage" is actually oblique. To an observer in the egg axis with 

 head toward the apical pole the spindles are inclined with upper poles 

 to right (dexiotropic) or to left (leiotropic), and the upper cell is obliquely 

 dextral or sinistral to the lower. This type of cleavage has been called 



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