214 CELLS, TISSUES, AND ORGANISMS 



the basic similarity between all types of cells in the chicks embryo, 

 when dispersed or freshly aggregated, seemed to bar further progress 

 with this issue. 



Heterologous cell aggregates 



The obstacle was overcome, and the analysis of the whole problem 

 was encouragingly advanced, by the finding that cells from mouse 

 embryos could be co-aggregated with cells from chick embryos and 

 compounded into chimaeric tissues ( Moscona, 1957, 1959a, 1961 ) . Due 

 to various morphological diflFerences between cells from these two 

 species (Wolff, 1954), they could be individually identified even when 

 completely commingled. Cells from one species thus served as markers 

 against those of the other. Simultaneously this system offered a means 

 for examining how generally valid the postulated premises were— 

 whether the cell properties involved were in principle comparable even 

 across generic differences. 



The practical expectation in taking up these heterologous systems 

 was that, within the intergeneric cell compounds, the cells would asso- 

 ciate in accordance with histological identities and kinships, irrespec- 

 tive of different generic derivations, i.e., that cells from like tissues— 

 though from different species— might join to form a mutual fabric, while 

 cells from different tissues would form separate groupings. If borne out, 

 this would not only confirm our premise but might also suggest further 

 means of inquiry, particularly into the nature of the cues and recogni- 

 tion mechanisms that guide cells in their histogenetic maneuvers. 



The following tests were carried out. First, co-aggregates were 

 compounded of mouse and chick cells derived from similar tissue ( het- 

 erologous-isotypic combinations ) , and their architecture was examined. 

 It was, indeed, found that cells from both species cooperated in con- 

 structing chimaeric tissues. For example, aggregates compounded of 

 intermingled neural-retina cells from chick and mouse embryos of 

 comparable stages of development consisted of tissue made up of cells 

 from both species ( Figure 12 ) . Evidently generic distance did not ad- 

 versely affect the ability of these histogenetically affiliated cells to es- 

 tablish mutually acceptable, chimaeric fabrics. This point was fully 

 borne out in other heterologous combinations in which it was possible 

 to match the cells as to functional kinship and stage of development. 

 For instance, intermingled cartilage-forming cells from both species 

 produced typically identifiable cartilage in which the cells from both 

 animals were closely interspersed and bound by a common matrix 

 (Figure 13). Such histochimaeras were maintained in culture for sev- 

 eral weeks without noticeable incompatibility between the cells. 



In another series of experiments the alternative situation was 



