88 BULLETIN: MUSEUM OF COMPAEATIVE ZOOLOGY. 



ditions found in his later work on the same material, in which the 

 centrosome and sphere were discovered. 



Heidenhain ('94) declared that the presence of a centrosome was least 

 to be expected in nerve cells. ISTevertheless, he pointed out that the 

 evidences of radial and concentric arrangement which have been de- 

 scribed in such cells demand that a careful seai'ch for the centrosome 

 he made. 



Flemming ('95) mentioned the frequent occurrence of the concentric 

 arrangement in the nerve cells of many animals investigated by him. 

 He says (p. 25) : " Die Anordnung ist nicht im vollen Sinne coucen- 

 trisch, sondern scheint von der Polstelle in zwei opponirten Strahlungen 

 an der Peripherie der Zelle zu verlaufen, so dass der Kern etwa die Mitte 

 zwischen beiden bildet." 



Von Lenhossek ('95), after the demonstration by Flemming and 

 Heidenhain of the centrosome in resting cells, returned to the study of 

 the nerve cells in which the excentric and flattened nuclei had been 

 observed, and by methods other than those used in his previous work 

 demonstrated the presence of a centrosome and sphere. The condi- 

 tions which he finds in some of the spinal-ganglion cells of the frog are 

 briefly as follows. 



The nucleus is in a very excentric position and generally flattened, or 

 even concave, on the side neai-est the cell centre. The centrosome and 

 sphere lie at the centre of the cytoplasmic mass, exclusive of the nucleus ; 

 for, as a result of planimetric measurements, von Lenhossek declares 

 that "das Centrosom ist also in den Spinalganglien des Frosches 

 wohl ein Centralgebilde in Bezug auf das Zellprotoplasma mit Abzug 

 des Kerns, nicht aber in Bezug auf die kernhaltige Gesammtzelle " 

 (p. 367). Heidenhain had found that in leucocytes the " Microcen- 

 trum " occupied the geometrical centre of the entire cell, except when 

 displaced by a nucleus greater than one-half the diameter of the cell. 



The centrosome of von Lenhossek is about 0.5 ^ in diameter, and is 

 composed of never fewer than twelve granules imbedded in a feebly 

 staining matrix, which he compares to the ** achromatische Substanz- 

 masse" by which Heidenhain's "Centralkorper" were connected. Sur- 

 rounding this group of granules is the sphere, which appears in sections 

 as a sharply defined, homogeneous, circular area from 4 /i to 6 fi in diame- 

 ter. Its sharp boundary is not due to a layer of microsomes, but is 

 marked only by the contrast between the substance of the sphere and 

 the surrounding cytoplasm. This contrast is often emphasized by a 

 narrow clear area about the sphere. 



