3o6 FINE-STRUCTURE OF PROTOPLASMIC DERIVATIVES HIT 



with a characteristically inflated sporangiophore, the sporangium 

 does not rotate. 



It follows from this discussion, that there must be a local growth by^ 

 intussusception, the mechanism of which is not yet fully understood. 



Crossed lamellar systems. The growth in area of membranes which; 

 are obviously cross-textured is equally difficult to explain. 



An instructive example of a microscopically laminated structure is. 

 provided by the inner layer of the lobster shell. As an entity, this, 

 layer behaves like a uniaxial, optically negative composite body; i.e.,. 

 seen trom the surface, it is isotropic. In cross-section, on the other 

 hand, strongly birefringent (positive with reference to the lamellation) 

 and isotropic layers are seen to alternate. Older investigators (Bieder- 

 MANN, 1903) thought these lamellae possessed cross-wise fibrillation 

 at right angles. Were this true, it should be possible to cut cross-^ 

 sections at 45° to the two fibrillar directions through the composite 

 body in which all the lamellae would show the same behaviour in 

 the polarizing microscope. This, however, is not the case, for cross- 

 sections, in whatever direction, through the lobster shell all invariably^ 

 disclose the same pattern of lamellation. Schmidt (1924, p. 238) 

 therefore assumes that the iibrillae in consecutive, very thin, parallel- 

 fibred layers very gradually change direction, so that two layers at 

 a certain distance from each other will contain fibrillae crossed at right 

 angles, but those in between will contain fibrillae in any of the 

 transitions from 0° to 90°. An arrangement such as this is indicated 

 in Fig. i48bi (p. 303). This should be verifiable optically for, in the 

 transition from lamella to lamella, the light retardation should drop- 

 following a sine curve from the maximum value to nil. X-ray analysis, 

 would likewise show whether all possible fibrillar directions are before 

 us. It seems to me an important point that the hypothetical layers are 

 submicroscopically thin for, were they of microscopical dimensions, 

 it would mean that this is a comparable case to the spiral texture of 

 cotton fibre ; that is to say, owing to the obliquely crossed layers, the 

 top view of the interior layer of the shell could not be isotropic, but 

 would have to transmit some light under all azimuths. 



Instead of assuming submicroscopic lamellae consisting of parallel 

 microfibrils superimposed in different directions of orientation (Fig^ 

 148b, p. 303), it would be equally plausible to picture the micro- 

 fibrils as interweaving. 



