96 PATTERNS AND PROBLEMS OF DEVELOPMENT 



ment of the morphological pattern appears beyond question. They are 

 evidently concerned in reconstitution of individuals in fission or after ex- 

 perimental isolation of parts. Moreover, it may at least be questioned 

 whether axial co-ordination of ciliary beat and definitely directed locomo- 

 tion are possible without a gradient pattern. 



SPONGES, COELENTERATES, AND CTENOPHORES 



SPONGES 



Oxygen uptake and CO2 production are, in general, greater in apical 

 than in basal pieces of the sponge commonly known as Grantia.^ Accord- 

 ing to Hyman and Bellamy (1922), the oscular region is galvanometrically 

 negative to more basal levels. In longitudinally split individuals of Gran- 

 tia and Leucoselenia and several other elongated species with single oscu- 

 lum, the rate of reduction of permanganate and the resulting depth of 

 color decrease from the osculum basipetally. 



HYDROZOA 



Gradients of early embryonic stages and their changes in the course of 

 development, as indicated by differential susceptibility and reduction of 

 permanganate and in some cases by the indophenol reaction, are known 

 for several species (Child, 1925a). The oocyte of the calyptoblast hydro- 

 zoan Phialidium gregarium is attached in the gonad by one pole, the future 

 vegetal pole, the free pole becoming the animal or apical pole (Fig. 2g,A). 

 Polar bodies form at the apical pole, the first cleavage furrow progresses 

 from it, and it becomes the apical pole of the early blastula and planula 

 and the high end of the primary gradient (Fig. 29, B, C, D). In the later 

 planula a second gradient appears at the original basal end (Fig. 29, D); 

 the primary gradient becomes less distinct; the planula, if in good condi- 

 tion, attaches itself by the original apical end; and the secondary basal 

 gradient becomes the hydranth-stem gradient (Fig. 29, E). In the later 

 development of the branching hydroid the growth form is sympodial, that 

 is, each new hydranth bud is only temporarily the apical member and be- 

 comes a lateral branch when the next bud appears (Child, i9i9c^). The 

 reversal of polarity and gradient in development of the hydranth from the 

 planula may be regarded as the first step in sympodial development ; the 

 first hydranth bud arises as a new axis from the basal regions of the plan- 



<> Oxygen uptake higher in eleven of thirteen determinations on different lots; CO2 produc- 

 tion higher in ten of eleven determinations (Hyman, 1925). 



