38 GENERAL EMBRYOLOGY 



chromatin, a name given it on account of the ease with which 

 it is colored with such dyes as hsematoxylin (logwood) or 

 carmine. This chromatin is ordinarily in the form of small 

 flakes or granules of variable size, termed chromioles (Eisen). 

 These are always distributed along the linin fibers, which stain 

 less readily and are therefore described as achromatic. . Some- 

 times a few large masses of chromatin called karyosomes may 

 be seen within the nucleus. Apparently the chromatin may 

 be rapidly dissolved or condensed, or even formed anew, so 

 that its visible amount and condition vary considerably from 

 time to time. 



The structural differentiation of the nucleus is frequently 

 complicated further by the presence of one or more bodies 

 called plasmosomes or nucleoli, which often superficially resem- 

 ble the karyosomes, at least morphologically, although chemic- 

 ally and physiologically they are quite unlike (Fig. 16). The 

 nucleoli are spheroidal bodies, staining densely, though not of 

 the same material as the true chromatin/ The karyosomes are 

 sometimes called chromatin nucleoli. The nucleoli vary con- 

 siderably in number, size, and form, and their significance in 

 the nucleus is not altogether understood; probably several 

 unlike bodies having different functions have been included 

 under this single term. 



Another organ of the cell is the centrosome. This is not 

 always to be seen for it may be lost from the cell at certain 

 times and reappear later. While commonly a cytoplasmic 

 structure lying just outside the nucleus, in some forms it is 

 an intra-nuclear organ and it is quite possible that primitively 

 it was an essential part of the nucleus, as it still is in many 

 Protozoa (Figs. 29, 30). The centrosome is a minute densely 

 staining granule or pair of granules, sometimes hardly larger 

 than the granules or microsomes of the cytoplasmic reticulum. 

 Occasionally it consists of several separate granules closely 

 associated. The cytoplasm in the neighborhood of the centro- 

 some and directly under its influence is ordinarily differentiated 

 as the archoplasm. This substance consists typically of two 

 portions. A medullary region forming a small spheroidal 



