34 



HISTOLOGY 



The first and second segmentation stages afford the best demonstra- 

 tion objects. The nucleus of these blastomeres is, in its resting stage, 



about fifty micra in diameter with an 

 oval to rounded contour. The karyo- 

 somes and plasmosomes stain deeply 

 and are variable in number; usually 

 three or four are present. The largest 

 one is generally a deeply staining body 

 with a vacuole of lighter staining ma- 

 terial through which are seen granular 

 strands of the chromatic material (Fig. 



37). 



The chromatin is distributed as a 

 series of fine particles throughout the 

 nucleus and is supported by a retic- 

 FIG: 37. Beginning of fourth cleavage di- ulum of tough, non-staining linin 



vision of oosperm of Unio Centrosomes h j h b brought OUt and made 



appearing on sides of nucleus. o 



visible by careful staining with eosm. 



As in many other resting cells, the strands of linin tend to start radially 

 from the largest nucleolus and stretch irregularly to the periphery of 

 the nucleus as a network. The nuclear membrane is sharp and well 

 developed. It stains with most dyes. 



The resting nucleus shows no trace of a centrosome inside of it or 

 outside in the cytoplasm. The first sign of an approaching division, 

 besides an increase in the staining power of the chromatin, is the sudden 

 appearance of a centrosome on each side of the nucleus. These struc- 

 tures are in the cytoplasm, but rest 

 closely against the nucleus. Each 

 centrosome consists of a large centri- 

 ole lying in a very small centrosphere 

 from which long, delicate rays pass 

 out into the cytoplasm. 



At this time the linin disappears 

 and the chromatin begins to gather 

 into larger masses (Fig. 38). The 

 nucleolus often, but not always, 

 shows much vacuolation. The cen- 

 trosomes grow in size and apparent 

 strength. 



Figure 39 shows the next impor- 

 tant step. The chromatin has formed 

 a very long and thin spireme, some parts of which are thicker than others. 

 The nucleolus is very much smaller, and the centrosomes are beginning 



FIG. 38. A slightly later stage than in 

 last figure. Chromatin gathering. But 

 one centrosome shown in this section. 



