58 



GENERAL EMBRYOLOGY 



times without any corresponding cytoplasmic divisions, until finally, all 

 at once, the cytoplasm is cut into a large number of small cells. In 

 these forms of division no mitotic figure is formed ordinarily, and one 

 of the modifications due to association with reproduction is seen in the 

 fact that the resulting bud, or brood-cell, may have a form and structure 

 entirely unlike that of the mother cell. In budding, especially in that 

 form called bud-fission, the nucleus does not ordinarily divide until 



A B 



FIG. 26. Nuclear division in the Ciliate, Dileptus, From Calkins, "Proto- 

 zoology." A. Vegetative form. Nucleus in the form of chromatin granules 

 scattered through the greater part of the cell ("distributed nucleus"). B. 

 During division. Each chromatin granule elongates and divides into two. 



after the bud is practically completed and ready to be cut off, i.e., 

 cytoplasmic division tends to precede nuclear division. Another com- 

 plication due to the same association is the frequent differentiation of 

 two forms of chromatic substance in the nucleus. These are the repro- 

 ductive chromatin, or idiochromatin, and the vegetative, or tropho- 

 chromatin (Dobell) ; in some Protozoa these may come to be organized 

 into two separate nuclei which are sometimes equivalent to what are 

 called the micro- and macronucleus respectively. We have already 

 mentioned the distributed nucleus of many unicellular organisms in 

 which the chromatin is not organized into a definite nuclear organ, 

 but is in the form of scattered granules or collections of granules 



