CLEAVAGE AND DIFFERENTIATION 



determinate and indeterminate I omit from the discussion 

 for two reasons. First, since in an indeterminate egg, as 

 that of an echinid, the initial cleavages are determinate, 

 it can not be called strictly indeterminate. Second, since, 

 all monoembryonic eggs finally become unipotent, we can 

 not say that only determinate eggs are unipotent. 



The evidence at hand shows clearly that in one respect 

 eggs are the same: I refer to the fact, never sufficiently 

 emphasized, that before fertilization all eggs, amenable to 

 the experiment of merogony, have capacity to produce 

 many embryos.^ If eggs in the fertilizable stage be broken 

 up into fragments and these be inseminated, each fragment 

 produces an embryo. The developmental capacity of such 

 egg-fragments is as great in the egg of Chaetopterus, in which 

 pluripotency is lost early during cleavage, as in that of 

 echinoderm eggs, in which pluripotency persists longer. 

 Some eggs normally give rise to many embryos, as that of 

 the Texas armadillo that always forms four, and those of 

 some insects which give rise each to hundreds. The fer- 

 tilizable egg thus is a system of multiplex embryonic 

 potency. This pluripotency however, except for normally 

 polyembryonic eggs, is never realized in the normal process 

 of development. Instead, the egg develops as a mono- 

 embryonic system which becomes successively restricted 

 within the domain of monoembryogenesis. It is in this 

 domain that our problem of differentiation during cleavage 

 lies.2 



The primary event of the cleavage period is the act of 

 cleavage, a rhythmical phenomenon. This sundering 

 involves both nucleus and cytoplasm; for although in some 

 cases the nucleus divides without cytoplasmic division at 

 one time or another during cleavage, normally no egg ever 



^ For literature on merogony see Deluge^ ^Sq8, i8gga and b. 

 - See also Just, ig^/b. 



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