1897) 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 
Coufled cece 
ee cathe Slomach cetis } segmentakcon 
Gikermee cells. pretaadle celis 
Ztvistoms 2O 
Crt of GrowER 
repromucltve celts 
brood Atvtstors 
[PRET EPG ces 
Cova or sfermalozon) 
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.! 
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 Jn this and the tables to follow we use the signs X to indicate segmentation, 
AA to indicate brood divisions, and || to indicate divisions alternating with growth. 
