GAMETOPHYTE AND SPOROPHYTE. 347 



686. Types of nuclear division. The nuclear figures in the 

 vegetative cells are usually different from those in the spore 

 mother cells. In the spore mother cells there are two types of 

 nuclear division, (i) The first division in the mother cell is 

 called heterotypic. The early stages of this division usually 

 extend over a longer period than the second, and the figures are 

 more complex. Before the chromosomes arrive at the nuclear 

 plate they are often in the form of rings, or tetrads, or in the 

 form of X, V, or Y, and the number is usually one half the num- 

 ber in the preceding cells of the sporophyte. (2) The homo- 

 typic division immediately follows the heterotypic and the figures 

 are simpler, often the chromosomes being of a hook form, or 

 sometimes much stouter than in the heterotypic division. In 

 the vegetative cells (sometimes called somatic cells, or body 

 cells in contrast with reproductive cells) there is another type, 

 called by some the vegetative type. The chromosomes here are 

 often in the form of the letter U, and the figures are much sim- 

 pler than in the heterotypic division. In the somatic cells of 

 the sporophyte, as stated above, the number of chromosomes is 

 double that found in the heterotypic and homotypic divisions of 

 the mother cells and in the somatic cells of the gametophyte, 

 Fig. 411 represents a late stage in the division of somatic cells 

 in the sporophyte of podophyllum. The root tips of various 

 plants as the onion, lily, etc., are excellent places in which to 

 study nuclear division in the somatic cells of the sporophyte. 



687. Comparison with animals. In animals there does not 

 seem to be anything which corresponds with the gametophyte of 

 plants unless the sperm cells and eggs themselves represent it. 

 Heterotypic and homotypic division with the accompanying 

 reduction of the number of the chromosomes takes place in ani- 

 mals usually in the mother cells of the sperms and eggs. At 

 the time of fertilization the number of chromosomes is doubled, 

 so that all the somatic cells (except in rare instances) from the 

 fertilized egg to the mother cells of sperms and eggs have the 

 doubled number of chromosomes. Reduction, therefore, takes 

 place in animals just prior to the formation of the gametes, while 



