300 PATTERNS AND PROBLEMS OF DEVELOPMENT 



egg and sperm are near death and, except for parthenogenetic eggs, in- 

 evitably die unless fertilization occurs. Naturally parthenogenetic eggs 

 show, in general, less extreme morphological differentiation than those 

 requiring fertilization (Child, 1915^, chap. xiii). The sperm cytoplasm 

 may not take part in the changes following fertilization; but when em- 

 bryonic development begins, the egg begins to lose its egg characteristics, 

 and sooner or later the resulting cells attain what is commonly called 

 "embryonic condition." The pattern of organization in the egg, however, 

 becomes the basis of embryonic pattern. The earher stages of embryonic 

 development appear to involve a considerable dedifferentiation from the 

 egg condition with progress of a new differentiation within the general egg 

 pattern. If these changes are not dedifferentiation, there is probably no 

 dedifferentiation in any cell. 



That developmental determination can often be experimentally altered 

 has been abundantly demonstrated. If such labile determination repre- 

 sents an early stage of differentiation, its alteration must be a dediffer- 

 entiation. Under certain conditions synthesis and accumulation, and 

 under other conditions decomposition and loss of a substance or sub- 

 stances, take place in cells. These changes appear to be differentiation 

 and dedifferentiation. 



The assumption that certain cells are undifferentiated because they 

 give rise to new organs in agamic and reconstitutional development is 

 open to criticism on two grounds. First, it ignores the changes in struc- 

 ture and behavior usually evident in these cells; second, it involves the 

 assumption that these cells have been insensitive to their physiological 

 environment during the preceding development but suddenly become sen- 

 sitive to conditions resulting in budding, fission, or reconstitution. The 

 epidermal cells of the begonia leaf are very different from embryonic cells 

 in structure and behavior. They have formed cellulose membranes and a 

 large vacuole and under the usual conditions would never divide again ; 

 but under the conditions initiating bud formation their cytoplasm changes 

 in structure and staining properties, they begin to divide and grow, cel- 

 lulose membranes disappear, and a gradient system with vegetative tip 

 in its high region develops and gives rise to all the structures of the plant 

 axis (pp. 17^19). Fission and reconstitution in ciliate protozoa apparently 

 involve extensive ectoplasmic dedifferentiation and redifferentiation. Old 

 cilia and cirri "melt down" into the ectoplasm, and new ones develop 

 from other regions." Either the ectoplasmic structures of these animals 



" E.g., Dembowska, 1925, 1926; Lund, 191 7. Many other papers give similar data. 



