296 O. VAN DER STRICHT 



the membrana reticularis and is fibrillar in structure, the fibrils 

 running parallel to the axis of the plate and in continuity with 

 those of the head. 



The so-called 'head' is formed of at least two segments, the 

 smaller superficial one being in close contact with the head of 

 the outer pillar. In the bat, the upper part appears to be 

 reduced, from compression between neighboring elements, to a 

 simple fibrillated lamella (fig. 13"'"'^, ihd), while in the lower 

 part there is a thin cytoplasmic layer lateral to the fibrils (fig. 13^, 

 ihd). In the white rat (fig. 17, ihd), the common rat (fig. 18, 

 ihd), and particularly in the cat (fig. 9, ihd) twelve days after 

 birth, this superficial lamella is obviously thicker, its lateral 

 cytoplasmic layer being larger. In the bat (fig. 13"^, ihd) and 

 other mammals this layer increases in breadth at the level of 

 the lower part of the head, whence, without any demarcation, it 

 blends with a larger, deeper segment. This is not connected 

 with the outer pillar, but is situated below the head of the 

 latter. It is a little shorter than the superficial segment and 

 gradually tapers and continues with the body (ipb) of the pillar. 



During the first stage of its development the future head of 

 the inner pillar is a four-sided, somewhat flattened prism (figs. 1, 

 2, and 3, ip), nearly uniform in diameter, although tapering to 

 its apex. It is composed of a granular or homogeneous cyto- 

 plasm and a bundle of fibrils, which occupy the medial side of 

 the lower part of the prism and the central area of its superficial 

 portion where, in the earliest stages of development, the fibrils 

 are arranged in the fonii of a hollow tubule (fig. 3, ip) which 

 later gives rise to a solid bundle. During the second stage of 

 development the future head undergoes no very marked changes. 

 By compression from the outer pillar head its superficial segment 

 becomes somewhat thinner — lamellar in shape (figs. 4 and 6, 

 ip) — while its lower segment maintains its previous size or 

 enlarges slightly in the neighborhood of the pillar body. At the 

 same time the transformations above mentioned are occurring in 

 its cytoplasm in the proximity of the obturator septa. In order 

 to clearly recognize the lamellar shape of the superficial segment 

 of the head, cross-sections are needed. A longitudinal fibrilla- 



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