640 PATTERNS AND PROBLEMS OF DEVELOPMENT 



bae continue to arrive and undergo change in condition. Presumably oxy- 

 gen tension is considerably lower in the interior or more basal parts of 

 the aggregate than at the free end. That this axis in its earlier stages is 

 a simple gradient in physiological condition of the constituent amoebae 

 and the cells developing from them appears highly probable. The change 

 from aqueous to aerial environment, as the aggregation rises above the 

 substrate, may also be a factor in bringing about the change in condition 

 in the cells; and its effect is probably, within limits, a function of time of 

 exposure. If this is the case, a longitudinal gradient in physiological con- 

 dition must result with the high end apical, where the active amoebae 

 are arriving. Moreover, the aggregation itself may determine an environ- 

 ment which alters the physiological condition of the amoebae; and any 

 such change probably progresses with time, so that older parts of the 

 aggregate have undergone more change than younger, and a gradient 

 results. In fact, it seems improbable that an aggregation such as occurs 

 in these forms is possible without development of a gradient pattern, and 

 the morphogenesis suggests such a pattern. 



The successive separations by constriction of masses of amoebae which 

 give rise to branches suggests a dominance with limited range at the 

 stages concerned and successive physiological isolations, as continued ar- 

 rivals of amoebae in the apical region elongate the sorogen. The physio- 

 logically isolated mass apparently undergoes some activation, as in other 

 organisms, forms local aggregations, and these develop as branches. Har- 

 per's photomicrographs of the branch-forming masses suggest a slight 

 local dominance at the nodes; that is, the forms of the masses (Fig. 204, 

 B) and in many cases differences in size of sori and lengths of stipes in a 

 single whorl (Fig. 204, D, E) suggest that the branches of a whorl do not 

 develop simultaneously but in sequence. 



This development differs in certain respects, though apparently not 

 fundamentally, from development of axiate pattern in cell aggregates of 

 sponges and hydroids (pp. 348, 418). Here the aggregation and the axiate 

 pattern result from a directed motor reaction and probably from differ- 

 ential exposure to certain environmental factors. In sponge and hydroid 

 aggregates aggregation supposedly results from chance contacts of cells, 

 and axiation is determined by a spatial environmental differential. That 

 either the myxamoebae or the dissociated sponge or hydroid cells possess 

 any inherent characteristics that enable them to originate an orderly and 

 definite multicellular axiate pattern independently of a chronological or 

 spatial differential in environmental factors is neither probable nor indi- 



