VI CHROMIDIA 193 



through in a state of solution. Various views are held regarding the 

 meaning of the chromidial formation. Following Hertwig, it has been 

 supposed (Popoff, 1907) that it is a means by which the mass relations 

 between nucleus and cytoplasm are restored, if for any reason the 

 quantity of chromatin relative to the cytoplasm has become too great. 

 Chromidia have also been supposed to give rise by degenerative trans- 

 formation to reserve food material such as yolk (Popoff, 1907 ; Paludina 

 oocyte : Moroff, 1909 ; Copepod ooc5d;e) or fat (Popoff, 19 10 ; Musca 

 fat cells). 



Others again have ascribed to them a much more important role, 

 supposing that they have a formative function, being in fact the inter- 

 mediaries through which the nucleus produces the necessary changes 

 in the cytoplasm to bring about the differentiation of indifferent 

 embryonic cells into the specialized cells — muscles, nerves, etc. — of the 

 soma. According to Goldschmidt, the originator of this view, chromidia 

 may give rise to cell structures by direct transformation into them. 

 Thus in oocyte I. they are transformed into yolk granules, in embryonic 

 cells into muscle fibrillae, zymogen granules of gland cells, etc. 



This view was founded partly on observation of muscle and gland 

 cells in Ascaris (Goldschmidt, 1905, 1910), which have not been supported 

 by subsequent observers on the same material, and partly on analogy 

 with certain Protista ; for example, Trypanosomes, where a darkly 

 staining body (" kinetonucleus ") which is in close anatomical relation 

 to the flagellum and therefore apparently concerned with the function 

 of locomotion, is supposed by many to have been derived from the nucleus 

 and to consist of chromatin. At present, however, we cannot be said 

 to possess reliable evidence of the direct transformation of chromidia 

 into functional cell structures, though it seems not unlikely that they 

 may by fatty degeneration be transformed into yolk, fat, etc. 



Goldschmidt, however, also allows of a different mode of action of 

 the chromidia, considering that besides giving rise to functional cell 

 structures by direct transformation into them, they are in other cases 

 the formative bodies under whose agency the cytoplasm is moulded 

 into its various forms ; in other words, the morphogenetic activity of 

 the nucleus, to which we have so often alluded, is not exerted directly 

 by the nucleus as a whole, but by portions emitted through the nuclear 

 membrane, as chromidia, into the cytoplasm which it is to mould. 

 Goldschmidt is thus led to distinguish between two kinds of nuclear 

 material, propagative and somatic, or to select one of the various terms 

 that have been proposed by different workers — idiochromatin and 

 trophochromatin. Idiochromatin is the idioplasm, which we have 

 sufficiently characterized already. Trophochromatin is derived from 

 the idioplasm, and is the intermediary by which the latter, the master 



o 



