8 



Myxopliycese 



commencement of that differentiation which in higher types has resulted 

 in the complete demarcation between nucleus and cytoplasm. Moreover, on 

 considering the great differences in external morphological features which 

 are exhibited by the various members of such a primitive group, it is not 

 surprising that there should be different grades of incipient nuclear differ- 

 entiation within the limits of the group. 



This incipient nucleus consists of an achromatic ground substance 



occupying the alveoli of a reticulum in which 

 are located minute granules, principally at the 

 angles of the meshes. There is no limiting 

 membrane to this structure, and in the more 

 primitive forms the reticulum is directly con- 

 tinuous with that which occurs in the rest of 

 the protoplast. Concurrently with the absence 

 of a limiting membrane there are in some forms 

 radiating processes of the achromatic ground 

 substance, which have been shown to extend 

 through the coloured portion of the protoplast 

 as far as the cell-wall. 



The granules disposed along the threads of 

 the mesh behave with stains somewhat after 

 the manner of chromatin, but the staining is 

 rather imperfect and less constant. They may 

 be considered as the nucleo-protein substance 

 which, in the course of the evolution of the 

 higher types of protoplast, has become true chromatin with decisive staining 

 properties. It is not at all improbable that these granules were in the first 

 instance merely protein reserves 1 , and they are most probably to be 

 identified with Btitschli's 'red granules 2 ' and Guillermond's 'corpuscules 

 metachromatiques.' 



No structures corresponding with nucleoli exist. 



In some of the Blue-green Algas (e.g. Chroococcus macrococcus) there is 

 a clear differentiation of this incipient nucleus (fig. 5), whereas in contrast 

 to this relatively advanced type, the most primitive of all the Myxophycean 

 protoplasts is perhaps that of Myxobactron in which a differentiation has not 

 yet been demonstrated (G. S. W., 12). The vacuoles described by certain 



1 The quantity of 'chromatin' described and figured by Gardner ('06) is so surprisingly 

 large that one wonders whether it is really chromatin or merely the accumulation of protein 

 reserves which behave very like chromatin with nuclear stains. Also, is not Gardner's 

 'chromatin' identical with Hegler's 'anabamin'? It seems probable that both observers were 

 dealing with the same substance notwithstanding the wide difference of interpretation. 



' These granules must not be confused with ' Biitschli's red corpuscles ' in Diatoms. 



Fig. 6. Cell-division in Chroococ- 

 cus turgidns (Kiitz.) Nag. The 

 drawing out in approximately 



shown. Stained: iodine-green- 

 fuchsin. x 1000 (after a draw- 

 ing by Miss Acton). 



