744 CALKINS. [Vol. XV. 



These chromosomes, as in Noctihica, divide longitudinally, and 

 daughter-spiremes are formed, after which the chromatin passes 

 again into the resting reticulum. In EuglypJia, therefore, the 

 chromatin-changes seem to be practically the same as in the 

 Metazoa, and the elements are even more highly differentiated 

 than in Noctihica, for the latter has no chromatin-reticulum nor 

 definite spireme. The aggregation of the chromatin elements 

 after division to form karyosomes in Noctihica is another indi- 

 cation of the more primitive nature of this form. Noctihica and 

 Eiiglyplia, therefore, may justly be considered as connecting 

 links, so far as chromatin is concerned, between the conditions 

 in Metazoa and the simplest Protozoa. 



2. Origin of Centrosome and Sphere. 



The origin of the metazoan centrosome and attraction-sphere 

 from simpler elements in Protozoa has been the subject of a 

 number of interesting theories. The most recent and the most 

 important of these have been maintained by Heidenhain ('94), 

 Lauterborn ('96), and Hertwig ('96). According to Heidenhain 

 the " central-spindle," as described by Hermann ('9l), is identical 

 with the spindle formed by the micronucleus of the Infusoria 

 after the latter has undergone a loss of chromatin and has 

 acquired a differentiated center — the centrosome. He regards 

 the nucleus of the metazoan cell as derived from the infusorian 

 macronucleus, while the mantle-fibers are new formations. Not 

 only does he compare the nucleus and centrosome with the 

 macro- and micronuclei of Infusoria, but he even makes this 

 comparison the basis of a theory in which he derives the 

 Metazoa from the Infusoria. The comparison of centrosome in 

 Metazoa with the micronucleus is not original with Heidenhain. 

 Butschli ('9i) had already proposed it, and Hertwig and Lauter- 

 born had adopted the same view. Lauterborn's recent attempt 

 to derive the "achromatic " structures of Metazoa from elements 

 in the Protozoa is even more ingenious than that of Heiden- 

 hain. The essential difference between the two theories is that 

 in the one case (Heidenhain) the centrosome has been derived 

 directly from the nucleus, while in the other (Lauterborn), 



