ELEMESTABY STEUCTUHE OF OEGAXIZED EDDIES. 13 



muscle, nerve, brain, and membranes, but also blood-vessels, 

 glands, pigments, c. If we examine it during the embryonic 

 state, while it is yet in the egg, we shall find that the whole 

 head is made up of cells which differ merely in their dimen- 

 sions ; those at the top of the head being very small, those 

 surrounding the eye a little larger, and those beneath still 

 larger. It is only at a later period, after still further deve- 

 lopment, that these cellules become transformed, some of them 

 into bone, others into blood, others into flesh, &c. 



48. Again, the growth of the body, the introduction of 

 various tissues, the change of form and structure, proceed in 

 such a manner as to give rise to several cavities, variously 

 combined among themselves, and each containing, at the end 

 of these transformations, peculiar organs, or peculiar systems 

 of organs. 



[ 49. " All organic tissues," says Dr. Schwann, " how- 

 ever different they may be, have one common principle of 

 development as their basis viz., the formation of cells ;* 

 that is to say, nature never unites molecules immediately into 

 a fibre, a tube, and so forth, but she always, in the first in- 

 stance, forms a round cell, or changes, where it is requisite, 

 cells into the various primary tissues in which they present 

 themselves in the adult state. The formation of the elementary 

 cells takes place, in the main points, in all the tissues accord- 

 ing to the same laws ; the farther formation and transforma- 

 tion of the cells is different in the different tissues. 



[ 50. " The primary phenomena of cells are the follow- 

 ing : there is first a structureless substance present (cyto- 

 blastema), which is either contained in pre-existing cells, or 

 exists on the outside of these. Within this, cell-nuclei gene- 

 rally first arise round or oval, spherical or flat corpuscles 

 which usually include one or two small dark points (nuclear- 

 corpuscules) . Around these cell-nuclei the cells are produced, 

 and in such wise that they at first closely surround the nuclei. 

 The cells expand by growth, and indeed by intussusception, 

 ind the same thing very commonly happens, for a certain 

 period, in regard to the nuclei. When the cells have attained 

 a certain stage of development, the nuclei generally disappear. 

 With reference to the place at which the new cells arise in 



* These observations have been confirmed by Wagner, Valentin, Kolli- 

 ker, Schleiden, Mohl, Niigeli, and others. ED. 



