36 COMPARATIVE ANATOMY CHAP. 



3. These loops arrange themselves in an equatorial plane between 

 the two attraction centres, in such a way that their free ends are 

 directed outwards and their angles inwards. Fine achromatine fibres 

 run to the attraction centres (Fig. 33, D). 



4. The chromatin loops split lengthwise, so that their number is 

 doubled (Fig. 33, E\ 



5. The one half of the chromatin loop which has thus arisen moves 

 towards one attraction centre, the other towards the opposite centre. 

 The halves thus move away from each other ; fibres of achromatin 

 stretch between them (Fig. 33, F). 



6. The chromatin loops of each side have moved quite near their 

 attraction centres. Their order now becomes irregular, and they 

 again unite into a single tangle, round which again a nuclear 

 membrane can be recognised (resting stage, Fig. 33, G and H). 



During the last stages, at the surface of the cell in a plane between 

 the two attraction centres, a circular furrow appears, which becomes 

 deeper and deeper, and finally, when the two new nuclei have reformed, 

 divides the cell into two halves, each with a new nucleus. 



The cells which arise in the animal body by repeated division of 

 the egg develop in various ways, but always in such a way that the 

 greater number of them remain bound together in a special manner, 

 forming the so-called tissues. Four chief sorts of such tissues can be 

 distinguished : 



1. Surface or epithelial tissue. 



2. Connective tissue. 



3. Nerve tissue. 



4. Muscle tissue. 



I. Epithelial Tissue. 



This is the simplest form of tissue, and, as Comparative Histology 

 and Histogeny teach, the most primitive complex into which cells can] 

 combine. It may therefore be correctly described as primitive tissue,] 

 from which all other tissues are derived. Even among the Protozoa! 

 epithelium-like combinations of cells occur, as, e.g., in Pblvox, where the! 

 individuals (cells) of a colony are placed side by side in a layer which, ! 

 like a spherical mantle, encloses a central cavity. Such a form is, in' 

 many Metazoa, the immediate result of the first division of the egg ; * 

 it is here called the Blastula. The epithelial character consists in the" 

 regular juxtaposition of cells into superficially extended membranes. \ 

 These cover the outer and inner surfaces of the animal. The cells of | 

 an epithelium lie either in a single layer side by side (unilaminar 

 epithelium)^ or several layers lie one above another (multilaminar 

 epithelium). Different sorts of epithelium are always distinguished j 

 according to the differences in the individual cells which form them. 

 Thus we speak of tesselated epithelium when the cells are flat, of | 

 columnar epithelium when the cells are cylindrical, and so on. The! 



