386 SYNOPSIS 



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23. Spirogyra. 



A multicellular filamentous unbranched alga : interstitial 

 growth : gonads equal and similar, b.ut gametes show 

 first indication of sexual differentiation 192 



b. Superficial aggregates. 



24. Monostroma. 



Cell-division takes place in two dimensions .... 199 



c. Solid aggregates. 



25. Ulva. 



Like Monostroma, but cell-division takes place in three 

 dimensions 201 



26. Laminaria. 



Illustrates maximum size and complexity of compara- 

 tively slightly differentiated cells ... .... 201 



IV. SOLID AGGREGATES IN WHICH COMPLEXITY is INCREASED 



BY A LIMITED AMOUNT OF CELL-DIFFERENTIATION. 



27. Nitella. 



Segmented axis : nodes and internodes : appendages 

 leaves and rhizoids : apical growth by binary fission of 

 apical cell accompanied by immediate division and dif- 

 ferentiation of newly-formed segmental cells : complex 

 gonads (ovaries and spermaries) : alternation of genera- 

 tions, a gamobium or sexual generation (the leafy plant) 

 alternating with an agamobium or asexual generation 

 (the pro-embryo) 204 



28. Hydra. 



Example of a simple diploblastic animal : cells arranged 

 in two layers (ecto- and endoderm) inclosing an enteron 

 which opens externally by the mouth : combination of 

 intra-cellular with extra-cellular or enteric digestion . . 219 



29. Bougainvillea. 



Example of a colony with diploblastic zooids which are 

 nutritive (hydranths) and reproductive (medusae) : differ- 

 entiation of a rudimentary mesoderm producing imper- 

 fect tripoblastic condition : central and peripheral nervous 

 system : alternation of generations, a gamobium (the 

 medusa) alternating with an agamobium (the hydroid 

 colony) ; significance of developmental stages oosperm 

 (unicellular), polyplast (multicellular but undifferentiated), 

 and planula (diploblastic) 234 



