DETAILS OF NUCLEAR DIVISION 223 



inner side of the sporogenous layer by the participation of cells 

 next the connective. While this is going on the sporogenous 

 cells are undergoing longitudinal division, giving rise to the 

 mass of cells shown in E and H, g. These are proven by their 

 subsequent behavior to be the grandmother cells of the micro- 

 spores or pollen grains. 



Details of Nuclear Divisidn. — ^For the details in the division 

 of microspore grandmother and mother cells we must now 

 turn to the lilies, where these processes have been very critically 

 studied by various investigators. In these plants, as in all 

 others, where sexual reproduction takes place, the nucleus of 

 the microspore grandmother cell has half of its structure from 

 the paternal and half from the maternal side, a constitution 

 that descended to it from the fertilized egg, where paternal 

 chromosomes joined with an equal number of maternal chromo- 

 somes in the act of fertilization, as we saw to be the case with 

 Osmunda. We shall now see that there is a remarkable simi- 

 larity between the ferns and flowering plants in the details of 

 nuclear division in spore grandmother and mother cells. 



In a lily, as the grandmother nuclei begin the first steps in 

 division one finds the nuclear reticulum resolving itself into 

 parallel strands (Fig. 128, 2); then the synaptic stage ensues 

 where the strands become indistinguishable (Fig. 128, 3). Soon 

 the mass loosens and the parallel strands again come into view 

 (Fig. 128, 4, and Fig. 129, A). In favorably stained prepara- 

 tions the strands appear built of alternating colored and colorless 

 bodies, as in Fig. 129, A, B, etc., the former called chromatin, 

 and the latter linin. The chromatin bodies are also called ids. 

 In the parallel strands the ids are seen to stand opposite each 

 other. The parallel strands now unite and appear as one (Fig. 

 128, 5, and Fig. 129, B and C), but soon they separate more 

 widely than before (Fig. 128, 6, and Fig. 129, D). The double 

 strands now become spHt up transversely into several segments 

 (Fig. 128, 8, and Fig. 130, A), and if we count the pair in one 

 segment as one, then there are half as many of these as there 

 are chromosomes appearing in the dividing nuclei of the vege- 



