8 FIRST LECTURE. 



our predecessors. Towards 1840, Schwann, the founder of 

 modern histology, erroneously ascribed the cell membrane as 

 a third essential constituent to every cell, so that the cell 

 would have two concentric envelopes, that of the vesicular 

 nucleus and the external one of the cell body. The still fre- 

 quently used name of "cell contents" is derived from that 

 period. 



It is impossible for any one to demonstrate where such a 

 membrane really begins ; that the surface of a cell protoplasm 

 in contact with the surrounding objects may, and, in fact, often 

 does become more solid, would not be denied by any one ac- 

 quainted with the great changeability of protoplasm. We 

 may only speak of a cell membrane when we are able to iso- 

 late the thing, and thus place it with certainty before the mi- 

 croscopist's eye. A smooth, sharp, dark line of demarcation 

 on a possibly strongly changed cell corpse gives us neverthe- 

 less no proof of a membrane. We shall find later, it is true, that 



the isolation of an envelope on a fat 

 cell, for instance, is very easy. Tak- 

 ing, by way of example, our Fig. 14, 

 the lateral surfaces of the cylindrical 

 structure a are provided with a cover- 



FiG. 14. — Cylindrical epithelium • i • i •  i i_ i 



from the small intestine of tne "ig which is certainly recognizable. 



rabbit; a. Side view of the cells A , .11 1 •. • ^1 



with the thickened and somewhat AbOVe, at the broad part, it IS Other- 

 elevated seam, which is permeated . TT t . , , , 



by p.,rous canals; b, view of the wise. Here the cell membrane is 



cells from above, whereby the ap- . , . . , 



enures of the porous canals appear wanting ; and a thicker covering piece, 



as small points. , . . . . . ... 



permeated by very delicate longitudi- 

 nal canals, overlays the protoplasm. We perceive a cell cap- 

 sule on the mammalial ovum (Fig. 5, 2), while a more youthful 

 ovulum (1) still appears membraneless. In cartilage tissue- 

 cell capsules are quite ordinary occurrences ; we shall there 

 be more intimately occupied with them. 



We proceed further; we inquire after the life of the cell. 

 A life we have already ascribed to it, although a limited one 

 in the service of the whole. 



Can this, however, be demonstrated ? This question is 

 asked by many. We answer, yes. We recall to mind that 



