62 



GENERAL EMBRYOLOGY 



intranuclear structure which later moves out into the cytoplasm along- 

 side the nucleus. These conditions indicate strongly the nuclear origin 

 of the centrosome. And there is some reason for believing the spindle 

 also originally a nuclear structure, as it still is, in part at least. The 

 spindle is a less constant organ than the centrosome, compared with 

 which it is of secondary importance. In several Protozoa and simple 

 plants the spindle is entirely absent, usually where the centrosome is 

 intranuclear, so that no definite mitotic figure is formed. In other forms 

 the spindle is intranuclear, and then the centrosomes or their equivalents 

 may be absent, as in Opalina (Fig. 31). This form is further remark- 

 able for the chromatic character of some of its spindle fibers, and in 



FIG. 31. Nuclear division in the Infusorian, Opalina intestinalis. After 

 Metcalf. X 1200. A. Nucleus in anaphase showing chromatic reticulum, 

 which may be regarded as equivalent to the spindle, and branched, amoeboid 

 chromosomes. B. Late telophase. 



that, in the absence of centrosomes, the chromosomes separate by an 

 active amoeboid movement, suggesting a possibly primitive behavior 

 of the chromosomes in cell division (Metcalf) . 



It is of course impossible now to get definite information regarding 

 the evolution of the cell organs and the process of mitosis, and in these 

 connections the conditions found at present among the Protozoa are 

 only suggestive. Many of these conditions do suggest strongly, how- 

 ever, that the nucleus has been developed from the gradual aggregation 

 of scattered, chemically differentiated particles ; that within the nucleus 

 certain chromatic elements became specialized into division centers 

 which finally became extranuclear, achromatic bodies the centro- 



