1899 - 1900 .] Mr Crawford on Rectal Gland of Elasmobranchs. 57 
1. The outer layer, on which appears externally a coating of 
somewhat cubical peritoneal cells, is made up of bands of white 
fibrous tissue interwoven irregularly with a considerable amount 
of noil-striated muscle-fibre running in a circular and longitudinal 
direction. In this tissue are to be found at intervals large sinuses 
of an irregular shape, lined with endothelial cells, and containing 
blood-corpuscles. Towards the inner part the muscle-fibres become 
closer, forming a definite band resembling a muscularis mucosae 
external to the glandular tissue of the middle layer. Under a high 
power there is nothing further to be remarked. 
2. The middle or glandular layer is composed of a number of 
few-branched tubules, radially arranged, separated by capillaries, 
which are usually gorged with blood. Under a high power the 
gland cells are seen to be cubical, mono-nucleated, ill-defined from 
one another, and of a granular appearance. This latter is due to 
the protoplasmic network and not to the presence of any foreign 
substance. The iron and haematoxylin staining method recom- 
mended by Heidenhain for showing zymin granules gave here a 
negative result. The nuclei of the cells are large, possessing an 
evident nuclear membrane and nuclear network, and showing three 
to five nucleoli. The fibrous tissue of this layer is slight, consisting 
of thin septa passing inwards from the outer fibro-muscular layer 
first described. 
3. The central layer begins at a varying distance from the 
periphery by the sudden transition of the gland cells into the 
epithelium of ducts, which open after a short course into the 
central lumen. 
Between these ducts and immediately external to the epithelium 
of the lumen are seen very large irregularly-shaped sinuses lined 
with endothelial cells, and filled with blood-corpuscles. 
The lumen of the organ is large, though it is often compressed 
so as to seem almost valvular ; in many cases it contains a granular 
substance of indefinite structure, which was unaffected by the 
staining reagents employed. 
On examining the layer under a high power, the epithelium of 
the ducts and lumen is seen to be of the type described as transi- 
tional, showing several layers of polygonal cells, flattened as they 
approach the free surface. In many cases the more superficial 
