CELL AND PROTOPLASM CONCEPTS 



15 



work of biologists during the past fifty 

 years has revealed a further complicated 

 organization of both nucleus and cj^toplasm 

 that would have greatly astonished earlier 

 students of the cell. 



It was early showTi that many animal 

 cells have no such membranes as plant cells 

 have. Briicke, Beale and Max Schultze at 

 nearly the same time, 1861, reached the con- 

 clusion that a structural cell membrane is 

 not a necessary part of a cell. With the 

 development of the protoplasm concept it 

 became evident that the name "cell" was a 

 misnomer and many attempts were made 

 to supplant it. Beale (1870) proposed the 

 name "bioplast," Sachs (1892) the term 

 "energid," Hanstein (1880) the name 

 "protoplast" as more truly descriptive of 

 what the cell really is. Fortunately, none 

 of these terms has been generally accepted. 

 Scientific names should be first of all per- 

 manent, and should not be changed every 

 time new or contradictory qualities are 

 found in the objects named. 



IV. The Organization of Protoplasm 



Nothing else has so fully revealed the 

 complex organization of protoplasm as have 

 the detailed and long-continued studies on 

 nuclear and cell division. In 1835, von 

 Mohl described cell division in a filamen- 

 tous alga, Cladophora, in which a circular 

 constriction around the middle of the cell 

 deepened until the cell was cut in two. In 

 1841, Remak described the division of red 

 blood corpuscles of the embryo chick in 

 which the nucleolus first divided by con- 

 striction, then the nucleus and finally the 

 cell body. For a long time it was supposed 

 that this "type of Remak" was a common 

 form of cell division, but Wilson, in 1924, 

 rightly says that it is one of the rarest. It 

 does occur in certain cases of direct divi- 

 sion (amitosis) as in the follicle cells sur- 

 rounding the eggs of certain insects, but in 

 most cases of direct division the nucleolus 

 does not divide and frequently the cell 

 body does not. Direct division, segmenta- 

 tion or fragmentation of the nucleus have 

 been described by many students of the 

 cell but such forms of nuclear division 

 probably occur only in fully differentiated 



tissue cells, e.g., in functional muscle or 

 gland cells, in the trophic nuclei of certain 

 protozoa, or in senescent and degenerating 

 cells. Many so-called cases of direct nu- 

 clear division are really stages of incom- 

 plete union of chromosomal vesicles follow- 

 ing indirect nuclear division (mitosis). 



Star-shaped figures had been seen by 

 C. G. Carus in the eggs of Unio as far back 

 as 1832, and by Grube in the cleavage cells 

 of Clepsine in 1844; a double star or di- 

 aster was seen by Krohn in the eggs of the 

 ascidian Phallusia in 1852 (c/. Mark 1881). 

 In 1873, Anton Schneider observed in the 

 eggs of certain flatworms that the vesicular 

 nucleus is transformed during division into 

 a mass of filaments (named chromosomes 

 by Waldeyer in 1890) ; he saw these fila- 

 ments separate toward two asters in the 

 cell and then disappear, after which new 

 vesicular nuclei appear in their places. 



Many investigators saw the nucleus dis- 

 appear and two new ones appear later, 

 but Strasburger, in 1875, was probably the 

 first to trace the continuity between the 

 disappearing mother nucleus and the two 

 daughter nuclei, through the formation, di- . 

 vision and separation of nuclear filaments 

 (chromosomes). In 1879, Schleicher ob- 

 served in living cartilage cells of the frog 

 that rods appear in the nucleus, the nuclear 

 membrane and nucleoli disappear, the 

 nuclear rods become arranged in a star- 

 shaped figure on a spindle, and then divide 

 and move to the two poles. Owing to the ' 

 movements of the nucleus in this process 

 he called it karyokinesis. 



In a series of important papers beginning 

 in 1878 and culminating in 1882, Flemming 

 added enormously to our knowledge of 

 nuclear division. The various stages of 

 this process were accurately traced in epi- 

 thelial cells of the salamander, and many of 

 the names introduced by him, including the 

 term mitosis for this form of nuclear divi- 

 sion, are still in current use. In 1884, 

 Flemming, Strasburger, Van Beneden and 

 Rabl found that the nuclear filaments 

 (chromosomes) double in number by longi- 

 tudinal division and Strasburger saw the 

 two halves of each migrate to opposite 

 poles. They also found that the numbers of 



