PROGRESS IN CYTOLOGY 177 



termed them, connected by protoplasmic threads, and 

 that growth resulted from the formation of new dermato- 

 somes from the included protoplasm. In the last years 

 of the century the intussusception theory had regained 

 much of the support it had lost, more especially when 

 Strasburger, in 1891, to a large extent abandoned his 

 previous views on apposition. 



The chemistry of the wall was carefully studied by 

 Mangin in 1892. Previous to that date it was generally 

 held that the wall, at least in its juvenile condition, was 

 composed of cellulose chiefly, and that the middle lamella 

 between any two cells was formed of pectic compounds. 

 It was also recognised that the cellulose became altered 

 by the deposition in it of suberin, cutin, or lignin, or by 

 its transformation into these bodies. Mangin's work 

 dealt chiefly with the pectic constituents, some' of which 

 he showed were neutral in reaction, such as pectose and 

 pectin, while others were acids, such as pectic and meta- 

 pectic acids. In cell division, according to Mangin, the 

 first appearance of the new cell wall is a film of pectates, 

 on both sides of which layers of cellulose and pectose com- 

 pounds are deposited. De Bary, in 1864, pointed out that 

 the walls of fungal hyphae did not give the well-known 

 blue reaction with iodine and sulphuric acid so character- 

 istic of cellulose, and Wisselingh, in 1898, showed that 

 in Fungi the cell walls are composed chiefly of chitin. 



THE ORIGIN AND DEVELOPMENT OF TISSUES 



After the establishment of the cell theory attempts 

 were made to follow the various changes that took place 

 in the differentiation of the primitive homogeneous cells 

 of the embryo and of the growing points into permanent 

 tissues. I have already drawn your attention to the 

 work accomplished by Naegeli in 1858, and by Sanio in 

 1863, in this relation, but it is to Hanstein that we owe, 

 in 1868, the first clear account of this subject. He 



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