No. 3.] THE PROTOZOA AND METAZOA. 753 



division of the nucleus, this " Nebenkorper " breaks up into 

 smaller masses, each of which becomes associated with one of 

 the daughter-nuclei. Here it forms a central-spindle, and the 

 spore-nuclei divide by mitosis, in the same way, apparently, as 

 in Noctiluca. A centrosome is not described. 



A review of the foregoing facts shows that different forms 

 of Protozoa present a nearly complete series in which may be 

 traced the possible development of the complex extra-nuclear 

 centrosome and sphere from an intra-nuclear "achromatic" 

 body. This, — the homologue of the attraction-sphere, — 

 apparently, first appears as an axial nuclear rod in Amoeba 

 crystalligera and Euglena viridis. In Spirochotia, Kentrochona, 

 and the micronuclei of Infusoria it is seen as aggregations of 

 "achromatin" — pole-plates — at the spindle poles. In Acthio- 

 spliacrium the pole-plates remain intra-nuclear for the greater 

 part of mitosis, but according to Brauer a portion of them at 

 least becomes extra-nuclear for a brief period ; and during this 

 period spindle-fibers are formed. The "achromatic "body remains 

 extra-nuclear in Paratnocba, in Noctiluca, and in the majority of 

 Metazoa. In Noctiluca and the Metazoa a portion of the " achro- 

 matic " body, — the central-spindle, — although extra-nuclear, 

 comes to lie during mitosis in the center of the nuclear plate, 

 where it occupies the same position, morphologically, that it 

 occupies in Amoeba crystalligera or Euglena viridis. 



The centrosome is not so easily traced, and it may be found 

 that in the majority of Protozoa the intra-nuclear so-called 

 " centrosome " is quite a different structure from the centro- 

 some of the Metazoa, although even here the centrosome has 

 been traced to an intra-nuclear position by many observers. 

 Its nuclear origin in Protozoa is made out by Balbiani in 

 Spirochofta and by Doflein in Kentrochona, while Schaudinn 

 describes a body in Amoeba crystalligera which must be homol- 

 ogous. In one case at least — Spirockona (Balbiani) — its 

 direct morphological connection with the chromatin has been 

 traced, although not conclusively.^ 



' In this case the centrosome relation can be questioned on account of the 

 place of origin of the supposed centrosome. In most cases the centrosome is 

 found in the region of the spindle poles, but in Spirockona, according to Balbiani, 



