549 



The constriction may divide the nucleus into equal or nearly 

 equal parts as in Figure 2 A, Figure 8 b, and many other cases 

 shown, or it may separate only a very small portion (Figs. 7 b, 

 8 a and c). Cases of division into three parts are not at all un- 

 common (Fig. 5 b and 7 a), but in such cases one division is usually 

 more advanced than the other, hence they are doubtless to be regarded 

 as merely the result of very rapid division. Occasionally the regions 

 marked off by the constriction are slightly different in color, one of 

 them, usually the one containing the small nucleolus, being very 

 slightly darker than the other but still homogeneous in appearance. 

 This difference is always very slight and no attempt has been made 

 to indicate it in the figures. 



The two parts of the dividing nucleus do not move apart as the 

 constriction deepens but remain for some time in close contact so that 

 in later stages a "nuclear plate" is formed between them. This form 

 of araitosis has been described by several authors but reference to the 

 literature is not necessary at present. Later separation occurs but 

 even after separation the parts which were in contact may retain the 

 flattened form for a longer or shorter time; Figure lie represents a 

 case of this kind. Cases like Figure lid of two nuclei lying near 

 together, one containing a large nucleolus, the other one much smaller, 

 are of frequent occurrence and are without doubt the results of recently 

 completed division. In the earlier stages of development all the organs 

 are syncytial in character so that the formation of a cell membrane 

 or the aggregation of a special mass of cytoplasm about each nucleus 

 does not necessarily follow the nuclear, division. 



All the figures except Figures 1 and 11 are taken from the 

 ovaries and testes, but the method of division in the other tissues 

 does not differ so far as can be observed from that in these organs. 

 Kapidly dividing nuclei as in Figure 1 may become much reduced in size. 



The Development of the Reproductive Organs. 



The earliest stages in the development of the reproductive organs 



appear as aggregations of rapidly dividing nuclei in the parenchyma 



situated on the inner (median) side of the longitudinal nephridial canals 



on each side of the proglottid. Figure P) represents a part of one 



1) In this figure and in Figures 2, 3, and 7 the more deeply stained 

 aggregations of cytoplasm about the nuclei are indicated, but the shading 

 is somewhat deeper than in the sections themselves. The fibrillae of 

 the parenchyma are represented schematically and the almost transparent 

 matrix in which cells and fibres are imbedded is indicated by slight shading. 



