283 REMAK, ON THE EMBRYOLOGICAL 



animal cells, such a well-marked distinction is set up between 

 the membrane and contents as is met with in the vegetable 

 kingdom, or whether this separation be not, rather, confined 

 to those cases in which, as in plants, it is intended to afford 

 the cell a firmer protection, or to regulate the entrance of 

 nutritive materials through a solid partition. By no means, 

 therefore, can it be asserted that the foregoing observations 

 afford grounds for a general decision with respect to all kinds 

 of animal cell membranes. The more closely we examine 

 the elements of the data already obtained, the more essential 

 does it become that we should be able to determine the real 

 conditions of the membranes in those classes of cells of which 

 the tissues themselves are formed. So long as in the mem- 

 branes of the embryonic cells Ave possess only chemical means 

 of exhibiting it, and no other means of distinguishing its exist- 

 ence at all comparable in clearness and certainty with those 

 afibrded us in the vegetable cell-membrane, this difficult 

 problem can hardly be approached with any hope of success, 

 and we shall be confined, in the distinguishing between cell- 

 membranes, thickening layers, and intercellular substance, 

 to the deceptive and unsatisfactory appearances arising from 

 the varjdng states of aggregation of these parts. 



But if there actually be, as is said, no chemical difference 

 at all between membrane and protoplasm (a proposition, how- 

 ever, which still remains open to proof or to contradiction) what 

 comes of Schultze's assertion that a cell is nothing at all but 

 a " mass of protoplasm with a nucleus l!" 



In the next place, we must look at the history of the cell 

 theory. According to Schwann, the embryonic cells are said 

 to arise in the germ after the manner of crystals, and in fact 

 that the nucleus is first formed, around which the protoplasm 

 is stated to be afterwards deposited ; and in the same way new 

 cells arise, sometimes without and sometimes within cells, in 

 the latter case becoming free, it is said, by escaping, like a 

 nest of pill boxes, one from within another. 



After I had, as early as in 1841, observed the division of the 

 blood-cells, and in 1845 the longitudinal division of the mus- 

 cular fibre-cells, I satisfied myself in 1852, that the majority 

 of all known cells might be referred to the progressive division 

 of the original segmentation cells. 



But now arises the question, as to hosv this division is 

 effected. Is it owing to the protoplasm becoming soft and 

 breaking up at any given spot? or, as in the vegetable cell, is 

 it owing to the constriction of the cells from without inwards, 

 and the formation of septa, which are to be regarded as con- 

 tinuations of the cell membranes ? 



