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 years 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 progeny 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 blastulae 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 



