6o PATTERNS AND PROBLEMS OF DEVELOPMENT 



material for determinations of difference in respiratory rate along the 

 polar axis, but difficulties and more or less uncertainty are involved in all 

 such determinations. Such organisms are either fully developed or in an 

 advanced stage, and data from them can be no more than suggestive or 

 indicative of conditions in earlier developmental stages. They give no in- 

 formation concerning axial characteristics at the beginning of develop- 

 ment or of changes which may have occurred during its course. More- 

 over, some of them, such as planarians and annelids, have certain organs 

 at particular body-levels which may have respiratory rates different from 

 those of other parts of the body at the same level; also, differences in rate 

 along the gut may not be in the same direction as differences in the body 

 wall. In planarians differential susceptibility and rate of dye reduction 

 in the body wall decrease from the head posteriorly to the fission zone in 

 forms with a posterior zooid, but there is some evidence that the gradient 

 in the gut decreases in both directions from the pharyngeal region. If 

 respiratory rates parallel these differences, the respiratory rates of pieces 

 from different levels of the planarian body may not show the differences 

 which are actually present, because differences in the gut may more or less 

 balance differences in the body wall. In practice we may apparently de- 

 crease differences in the gut by starving the animals for a week or more, 

 but even with this precaution we cannot be certain that we obtain a true 

 picture of the respiratory differentials in the body; nevertheless, the fact 

 that differences in rate are usually absent in pieces of animals with food 

 in the gut and become definite and consistent in sufficiently starved ani- 

 mals appears in some degree significant (pp. io8, 737). The unbranched 

 stem (hydrocaulus) of the hydroid Tubularia, or Corymorpha, meets the 

 desired conditions, as far as differentiation of regions is concerned, as 

 nearly as any animal species ; but in the case of Tubularia the presence and 

 different thickness of the perisarc at different levels and in Corymorpha the 

 increase in volume of the entodermal parenchyma basipetally and the 

 presence of perisarc on the basal part of the stem show that even these 

 forms are not without possible complicating factors for respiratory deter- 

 minations (Child and Hyman, 1926; Hyman, 19260). 



Determinations or estimations of CO2 production also give some infor- 

 mation concerning regional differences in respiratory rate, but essentially 

 the same difficulties are involved as with determinations of oxygen con- 

 sumption. 



The chief methods available for determinations of oxygen consumption 

 bearing upon the question of a respiratory pattern in the individual are the 



