34 ILLINOIS BIOLOGICAL MONOGRAPHS [154 



removed from their hosts the intensity of the pigmentation does not appre- 

 ciably increase in the free state. Several such specimens were observed for 

 short periods and one female with little pigmentation except the dark ring 

 and bands was removed from the host on September 6, 1916, and kept alive 

 in an aquarium until the beginning of March, 1917, with no appreciable 

 increase in pigmentation. The specimen at that time died from an attack 

 of fungUu, 



Hypoderm. In the specimens five day old the hypoderm is already 

 clearly distinguishable as a layer of flattened cells, slightly thickened in the 

 region of the nerve cord, and lying just beneath the cuticula (Figs. 50, 51). 

 Partly by the thickening of the layer, but chiefly by the rapid multiplica- 

 tion of the cells, the latter have become cuboidal when the nine day stage 

 has been reached (Fig. 55). By a continuation of the multiplication and the 

 increase of the thickness of the layer the cells soon come to be columnar in 

 character. This condition is clearly shown at the ends of the specimens 

 in the twelve day stage, and appears over the entire body at slightly later 

 stages (Figs. 84, 86). Multiplication of the cells appears to be complete by 

 the time growth begins in the germ cells (Fig. 86) and further development 

 depends upon growth. In some cases there is a secondary flattening of the 

 cells before the development of the adult cuticula is initiated (Fig. 87), but 

 whether this secondary flattering occurs or the cells remain columnar 

 (Fig. 99), the small, round nuclei (Figs. 71, 73) become enlarged and 

 flattened in a direction parallel to the surface of the specimen (Figs. 42, 

 99). The enlargement and flattening occur by the flowing together of 

 several chromophil centers into one nucleus, the accumulation of achroma- 

 tic substance around these centers, and the development of a definite 

 nuclear membrane surrounding both (Figs. 70, 127). The chromatic 

 centers remain as distinct nucleoli within the larger nuclei. As the 

 adult stage is approached the nucleoli increase in size and become more 

 diffused so as to occupy more or less completely the entire space within 

 the nuclear membrane. At the same time the nucleus shrinks and becomes 

 excessively flattened, crowding together the nucleolar matter into a dense 

 mass (Fig. 124). 



Altho in cross sections of the hypoderm the cells appear to form a 

 syncytium with the cell boundaries merely indicated here and there 

 (Figs. 41, 73), tangetial sections and preparations of separated pieces 

 of hypoderm show distinctly the cell outlines (Figs. 127, 128). Such pre- 

 parations, however, show that the cell boundaries are not complete, the 

 cells remaining connected with each other by numerous protoplasmic 

 strands. During the earliest stages of the formation of the fibrous cuticula 

 there appears within the outer part of the hypoderm a system of canals 

 surrounding the cells (Fig. 41) and, at the intersections, sending out 

 branches to their bases. 



