. Text-book of normal histology: including an account of the development of the tissues and of the organs. ring the fifth month, of a solidepithelial club-shaped process from the stratum mucosum intothe primitive corium. About the seventh month a lumen appearswithin the tubular mass, an exit, however, for some time being Fig. 316. still wanting ; subsequently theobstructing epidermal layers arebroken through. Somewhatbefore the appearance of the lu-men the extremity of the cylin-der undergoes increased growth,resulting in the thickening andconvolution of the tube whichrepresents the future coi
. Text-book of normal histology: including an account of the development of the tissues and of the organs. ring the fifth month, of a solidepithelial club-shaped process from the stratum mucosum intothe primitive corium. About the seventh month a lumen appearswithin the tubular mass, an exit, however, for some time being Fig. 316. still wanting ; subsequently theobstructing epidermal layers arebroken through. Somewhatbefore the appearance of the lu-men the extremity of the cylin-der undergoes increased growth,resulting in the thickening andconvolution of the tube whichrepresents the future coileddivision of the gland ; the fullexpression of the characteristicconvoluted arrangement, how-ever, is not attained until shortlybefore birth. The musculartissue of the secretory tubes,situated between the basement-membrane and the lining epithe-lium, is present before the close of the ninth month ; its origin isas remarkable as its position, since the muscle-cells are derivedfrom the elements of the adjacent ectoderm. The basement-mem-brane and the fibrous sheath are contributions from the Section of skin of human foetus, showing devel-oping sweat-glands. The latter grow as epithelialcylinders from the stratum Malpighii of the epi-dermis into the underlying corium ; the character-istic coil appears later. 282 NORMAL HISTOLOGY. CHAPTER XVI. THE CENTRAL NERVOUS SYSTEM. THE MEMBRANES. The spinal cord and the brain are surrounded by their envelopingmembranes, the dura, the arachnoid, and the pia; these affordadditional protection and support the blood-vessels in their course tothe nervous tissue. The dura consists of interlacing bundles of dense fibre-elastictissue, in the interspaces between which lie numerous plate-likeconnective-tissue cells ; many irregular granular elements resem-bling plasma-cells occupy the more superficial layers. The narrowclefts between the fibrous bundles represent lymph-spaces. The smooth unattached surfaces of the dura are clothed with a s
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