48o SYNOPSIS 



PAGE 



22. Agart'cus. 



Complexity attained by interweaving of hyphae in a de- 

 finite form : illustrates maximum complexity of a linear 

 aggregate 191 



23. Spirog^'ra. 



A multicellular filamentous unbranched alga : interstitial 

 growth : gonads equal and similar, but gametes show 

 first indications of sexual difTerentiation 194 



b. Superficial aggregate. 



24. Monostroma. 



Cell-division takes place in two dimensions 201 



c. Solid aggregate. 



25. Ulva. 



Like Monostroma, but cell-division taxes place in three 

 dimensions 203 



IV. — Solid Aggregates in which Compi.exitv is increased 



BY A LIMITED AMOUNT OK CeLL-DiKFERENTIATION. 



26. 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) 203 



27. Hydra. 



Example of a simple diploblastic animal : cells arranged 

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

 which opens externally by the mouth : combination of 

 intra-cellular with extra-cellular or enteric digestion . . 218 



28. Bougainvillca. 



Example of a cSlony 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 undifferenti- 

 ated), and planula (diploblastic) 234 



