40 ILLINOIS BIOLOGICAL MONOGRAPHS [160 



cloaca. These fibers are intermediate in structure between muscle fibers 

 and parenchyma cells. They are elongated, closely compact, but do not 

 stain as deeply as muscle cells and possess walls nearly as heavy as those 

 of the parenchyma. 



Parenchyma and mesenteries. The parenchyma arises first as spindle- 

 shaped or multipolar mesenchyme cells (Figs. 63, 67-69). Most of the 

 cells appear to arise at the ends of the body, but a few cells are also found 

 in the intermediate region at a very early stage and may arise there. In 

 the twelve day stage many of the cells are not distinguishable from muscle 

 cells between which they are frequently inserted, and even in much later 

 stages the distinction between the two kinds of cells is not always clear. 

 The cells remain generalized for a long time and are scarcely distinguishable 

 from cells that form the rudiments of other organs (Fig. 109). 



The multiplication of the cells takes place very rapidly. In the male 

 they often fill completely the spaces between the organs. Except at the 

 ends, where they form a solid mass from the beginning, they first form an 

 irregular layer lining the muscles and the nerve cord and forming triradiate 

 septa which enclose the germ cells. Ventral to the germ cells the cavity 

 still remains and the intestine is attached to the layer of cells over the nerve 

 cord (Fig. 75). By further multiplication of the cells the germ cells become 

 farther removed from the muscles and all or nearly all of the space becomes 

 invaded (Figs. 107, 108). 



In the females the multiplication of the cells is not so prolific. The 

 layer lining the muscles and nerve cord and holding the intestine in place 

 is formed as in the males, but it passes between the muscles and the ovaries 

 only at the end of the body, leaving the ovaries in contact with the muscles 

 thruout nearly their whole length. A few mesenchyma cells enter between 

 the ovaries and others are scattered thruout the body at different places, 

 but there are no definite layers enclosing the germ cells (Figs. 76, 79, 86). 

 By the later growth of the eggs nearly all of the spaces in the body are 

 eliminated. 



A short time before the adult cuticula is formed the cells become sur- 

 rounded by heavy layers of a hyaline substance that is stained with aniline 

 blue in Mallory's connective tissue stain. The cells then lie in cuboidal, 

 rounded, or polygonal chambers completely isolated from each other (Figs. 

 112, 116). Very soon the cells become shriveled, leaving only the heavy 

 walls with here and there a fragment of a nucleus or of cytoplasm. 



The layer of parenchyma immediately surrounding the nerve cord 

 deserves special mention on account of its peculiar mode of development. 

 In early stages, when the nerve cord is merely a thickening in the hypoderm, 

 this layer is continuous with the muscle layer lining the rest of the hypo- 

 derm and can at first not be distinguished from that layer (Fig. 84). Even 

 when fully developed these parenchyma cells are narrow and very much 



