From them are developed two distinct types of histological elements; the genital cells and the cnidoblasts or mothercells of the nematocysts.
Protohydra is a marine genus characterized by the absence of tentacles, by a great similarity to Hydra in histological structure, and by reproduction by transverse fission.
The use of the term to mean the individualized nucleated mass of living protoplasm, which, whether with or without a limiting membrane, primitively forms the proximate histological element of the body of every organism, dates from the second quarter of the i9th century.
IFor the histology of the comparatively simple but in many respects aberrant Bog-mosses (Sphagnaceae), see BRYOPHYTA.] The stems of the other mosses resemble one another in their main histological features.
Thus the histological differentiation of the sporogonium of the higher mosses is one of considerable complexity; but there is here even less reason to suppose that these tissues have any homology (phylogenetic community of origin) with the similar ones met with in the higher plants.
The leaves, again, have special histological features adapted to the performance of their special functions.
Secondly, the histology of fossil plants, particularly woody plants of the carboniferous period, has been placed on a sound basis, assimilated with general histological doctrine, and has considerably enlarged our conceptions of plant anatomy as a whole, though again.
The early histological researches of botanists led them to the recognition of the vegetable cell, and the leading writers in the middle of the ~9th century pointed out the probable identity of Von Mohls protopiarm with the sarcode of zoologists.
It is only comparatively recently that the methods of histological investigation used by animal physiologists have been carefully and systematically applied to the study of the vegetable organisms. They have, however, been attended with wonderful results, and have revolutionized the whole study of vegetable structure.
The differentiation of the plants substance so indicated is, however, physiological only; there is no histological difference between the cells of these regions that can be associated with the several properties they possess.
There are, however, anatomical and histological differences to be seen at any rate at the extremes between the undoubted nephridia of Goodrich, Meyer and Lankester, and the coelomoducts of the same authors.
In the majority of the Megascolicidae each sac is provided with one or more diverticula, tubular or oval in form, of a slightly different histological character in the lining epithelium, and in them is invariably lodged the sperm.
The histological elements of this portion remain distinct both by transmitted light and in actual sections.
These posterior brain-lobes, which in all Heteronemertines are in direct continuity of tissue with the upper pair of principal lobes, cease to have this intimate connexion in the Metanemertini; and, although still constituted of (I) a ciliated duct, opening out externally, (2) nervous tissue surrounding it, and (3) histological elements distinctly different from the nervous, and most probably directly derived from the oesophageal outgrowths, they are nevertheless here no longer constantly situated behind the upper brain-lobes and directly connected with them, but are found sometimes behind, sometimes beside and sometimes before the brain-lobes.
Blochmann regards as coelomic, but it must be remembered that his interpretation rests largely on histological grounds, and at present embryological confirmation is wanting.
In the formation of these the tissues break down, and in course of time lose their characteristic histological features.
Hansemann's "anaplasia " hypothesis seeks to find an explanation of the formation of new growths in the absence of the histological differentiation of the cell associated with a corresponding increase in its proliferative power and a suspension, or loss, of its functional activity.
From the histological examination of tumour cells there is no evidence to show that they resemble the protozoal unicellular organisms in occasionally passing through a sexual process of reproduction, i.e.
It has certain histological resemblances with the Nematoda and certain primitive Annelids, but little stress must be laid on these.
It is based upon the fact that the histological differentiation of the epidermis of their root is that generally characteristic of Monocotyledons, whilst they have two cotyledons - the old view of the epiblast as a second cotyledon in Gramineae being adopted.
The tissues and fluids are treated by various histological methods, but, to speak generally, examination is made either in films smeared on thin cover-glasses and allowed to dry, or in thin sections cut by the microtome after suitable fixation and hardening of the tissue.
Most of the specimens had formed a zone of secondary wood and phloem resembling the corresponding tissues in a recent Cycad; the similarity extended to minute histological details, as is shown especially in H.