1897J FUNDAMENTAL PRINCIPLES OF HEREDITY 237 



Protists ; for here the colonial organism can only be propagated by 

 the co-operation of all three kinds of cells. The individual cell is 

 no longer a Jack-of-all-trades, but it has been so specialised that it 

 needs the association and co-operation of cells specialised in other 

 directions to form a complete self-sufficing organism ; and each kind 

 of cell can by growth and division only reproduce its own type and 

 tissue ; but not the complete organism of which it formed a part. This 

 has been aptly termed by Orpen Bower a process of sterilisation. 

 We have noted the richer endowment of certain of the inter- 

 mediate-cells. We must now follow up the fate of the coupled-cell 

 (fertilised egg, oosperm). This divides afresh repeatedly, and by 

 its segmentation gives rise to a hollow spherical colony, one hemi- 

 sphere being composed of smooth cells, while the other is provided 

 with lashes. The latter now sinks into the former so as to give 

 the colony the form of a lined skull-cap. The lining is composed 





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of collared-cells, which are the stomach-cells; the outer layer of 

 cells again divides into two layers, the epidermic and middle 

 cells respectively. This is essentially the processes of reproduction and 

 early embryonic found growth in all Higher Animals, save that the 

 middle layer may be formed from the inturned cells instead of, or as 

 well as, the outer ones, and that the reproductive cells may be formed 

 in different layers in different classes. The annexed genealogical 

 table, starting with the coupled-cell and ending with the pairing or 

 sexual cells, represents the cellular pedigree in a Sponge. 1 



From the above it is clear that the coupled-cells, though they 

 are descended from middle-cells only, yet produce by their divisions 

 offspring that ultimately become cells of kinds which are different, 

 and have never been in the line of their direct descent. We might 

 compare this with a race of which the older and the younger 



1 In this and the tables to follow we use the signs X to indicate segmentation, 

 A A to indicate brood divisions, and II to indicate divisions alternating with growth. 



