190 THE INDUCTIONS OF BIOLOGY. 



But now concerning cells out of which, variously modified, 

 obscured, and sometimes obliterated, tissues are formed, we 

 have to note a fact of much significance. Along with the 

 cell-doctrine as at first held, when attention was given to 

 the cell itself rather than to its contents, there went the 

 belief that each of these morphological units is structu- 

 rally separate from its neighbours. But since establishment 

 of the modern view that the essential element is the con- 

 tained protoplasm, histologists have discovered that there 

 are protoplasmic connexions between the contents of adjacent 

 cells. Though cursorily observed at earlier dates, it was not 

 until some twenty 3^ears ago that in plant-tissues these were 

 clearly shown to pass through openings in the cell- walls. It 

 is said that in some cases the openings are made, and the 

 junctions established, by a secondary process; but the impli- 

 cation is that usually these living links are left between 

 multiplying protoplasts; so that from the outset the proto- 

 plasm pervading the whole plant maintains its continuity. 

 More recently sundry zoologists have alleged that a like con- 

 tinuity exists in animals. Especially has this been main- 

 tained by Mr. Adam Sedgwick. Numerous observations made 

 on developing ova of fishes have led him to assert that in no 

 case do the multiplying cells so-called — blastomeres and 

 their progen}^ — become entirely separate. Their fission is in 

 all cases incomplete. A like continuity has been found in 

 the embryos of many Arthropods, and more recently in the 

 segmenting eggs and blastulas of Echinoderms. The syn- 

 cytium thus formed is held by Mr. Sedgwick to be main- 

 tained in adult life, and in this belief he is in agreement 

 with sundry others. Bridges of protoplasm have been seen 

 between epithelium-cells, and it is maintained that cartilage- 

 cells, connective tissue cells, the cells forming muscle-fibres, 

 as well as nerve-cells, have protoplasmic unions. Nay, some 

 even assert that an ovum preserves a protoplasmic connexion 

 with the matrix in which it develops. 



A corollary of great significance may here be drawn. It 



