39 



furnish ed v/ith long delicate gelatinous horns. As Senn^ 

 has shovm, the formation of the horns is lacking under ah- 

 normal life conditions, nutrient solutions, in the usual 

 concentration and rich in oxygen or highly concentrated 

 nutrient solutions without the addition of oxygen, while 

 the gelatine is formed as a uniform coating on all parts of 

 the colony. A similar arrestment in the differentiation of 

 the colonies might also he ohtained, under certain conditions 

 in the diatoms just mentioned, or in Pediastrum granulatum, 

 whose lamelliform colony is composed of polygonal cells 

 two-armed at the edge, etc. The same arrestment may he 

 studied further in the colonies of individuals lacking 

 membrane auch as those known as plasmodia in the Myxomycetest 

 When the fruit bodies of Bictyostelium mucoroides are being 

 formed under normal life conditions, a division of labor 

 appears among those individuals which have united themselves 

 into a Plasmodium (rather, a pseudoplasmodium) of such a 

 kind, that a part of the Amoeba mass is used for the forma- 

 (42)tion of a stem, and the rest is transformed into spores.* 

 Recently the interesting evidence has been produced by Potts 

 that 3 under certain abnormal conditions this differentia- 

 tion is omitted, that under water, as well as on concentra- 

 ted nutritive agar ( 5.5. per cent. KIO3 ) the whole amoeba 

 mass is transformed into spores and that, conversely, in 

 development under a layer of oil, sterile stem cells are 

 formed without exception. Consequently, first one and then 

 the other process of differentiation is eliminated from the 

 course of development of the cell aggregate. 



As far as the differentiation of the tissues of the 

 multicellular grov/ths is concerned, it may be said, regard- 

 less of their diversity, that there is, in general, no organ 

 whose tissues could not be arrested in their differentiation 

 by factors acting more or less energe-trically. Thus, as 

 before, we will limite outselves to the treatment here of 

 a few tissue forms. 



Observations on the arrestment of tissue differentia- 

 tion may be made so easily, without troublesome experiments 

 that there exists a real superfluity of reports on this sub- 

 ject. In listing the authors, a choice will therefore suf- 

 fice, especially as many of these reports give inadequate 

 data. 



From the list of thallophytes and cellular cryptogams 

 the Uarchantiaceae furnish an instructive example. The 

 structure of the normally developed thallus is well known. 

 f43)Its elements are at the left in figure 10; an epidermis 



1. Ueber einige koloniesbild. einzellige Algen. 

 Bot. Zeitung, 1899, Bd. LVII , p. 39. 



2. Brefeld, Untersuch. aus. d. Gesamtgebiet d. 

 Mykologie, Heft VI, oBbel, Organographie. 1898, p. 21. 



^, Zur. Phyp;j.ologie des Dictyostelium mucoroides. 

 Flora, i^02, Bd/jCCI (ErganBungsbd.) p. 281, 



