224 BIOLOGICAL LECTURES. 



some forms (simple flagellates) it retains its definite shape, 

 although Schaudinn's observations on Oxyrrhis marina (a 

 flagellate with intranuclear sphere like that of Euglend] indi- 

 cate that the size and condition of this body are dependent upon 

 the density of the surrounding medium. When this flagellate 

 is placed in diluted sea water, the intranuclear body swells to 

 many times its normal size, and may even be forced out of the 

 nucleus, where, in the cytoplasm, it forms an abnormally large 

 spindle ; in concentrated sea water, on the other hand, it shrinks 

 to an abnormally small intranuclear granule. This important 

 observation opens the way for an explanation of many hitherto 

 unexplained structures. The nucleus of Amoeba proteus, for 

 example, contains chromatin in the form of minute granules, 

 which are arranged about the periphery of the nucleus, while 

 the central portion is occupied by a large homogeneous mass, 

 which can be explained as an enlarged or diffuse intranuclear 

 sphere. The pole plates also, which are widely distributed 

 throughout the Protozoa, may be explained as a temporary 

 accumulation of this ordinarily diffuse archoplasmic substance, 1 

 and thus homologous with the "nucleolus-centrosome" or sphere 

 of Euglena and with the centrosphere of Metazoa. 



The origin of the centrosome is far more complicated, and 

 cannot be conceived in so clear a way as the central spindle, 

 owing to the diverse observations and the complex theories 

 which have been made to explain it. Certain data, neverthe- 

 less, have been obtained which may serve as a basis for further 

 considerations. The process of mitosis in Actinosph&rium is 

 extremely interesting in this connection, and, observed by 

 Gruber, Hertwig, Brauer, and recently made the subject of an 

 extensive re-investigation by Hertwig, it has become perhaps 

 the most familiar of protozoan mitoses. Both Brauer and 

 Hertwig describe centrosomes as transient structures in this 

 cell, appearing, according to the former, during the anaphase 

 of the mitosis of encysted animals ; according to the latter, 

 before and during the two maturation divisions by which the 

 cells prepare for conjugation. Brauer's view as to the origin 

 of the centrosome and its fate is obscure and unsatisfactory, 



1 Cf. Hertwig. 



