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CELLS, TISSUES, AND ORGANISMS 



Figure 10. Section through a 

 24-hour aggregate of meso- 

 nephros cells from a 7-day 

 chick embryo. 



and identified as to origin. In the situations discussed above, where all 

 cells originated in one organ or in one tissue, this was, for obvious rea- 

 sons, not feasible. 



Heterotypic cell aggregates 



The simplest attempt to solve this difficulty was to mix together 

 cells from two quite different tissues— for instance, kidney-forming cells 

 with cartilage-forming cells— expecting that within the composite ag- 

 gregates thus obtained the tributory elements might be identified by 

 their architectural products. This expectation rested upon two assump- 

 tions. First, that such histogenetically alien cells, when lumped to- 

 gether, would not interfere with one another's activities so as to remain 

 chaotically conglomerated. Second, that the cells would retain their 

 original functional identities, cells from each source becoming assorted 

 and grouped independently of the others, so that an aggregate would 

 eventually contain two different tissues. The results of such experiments 

 conformed with these expectations (Moscona, 1956). Aggregates com- 

 pounded of interspersed cells from cartilage- and kidney-forming tis- 

 sues were indeed found to consist of both these types of tissue ( Figure 

 11). The different cells, though closely intermingled in the initial sus- 

 pension, were evidently able to become disentangled, sorted out, and 

 grouped according to histogenetic identities and properties. Cartilage 

 cells formed lumps of cartilage, usually in the center of aggregates; 

 kidney cells formed nephric epithelium, usually as an outer layer, with 

 an intermediate zone of connective tissue presumably of dual origin. 



