MUSEUM OF COMPARATIVE ZOOLOGY. 13 



The rhabditi which lie between the hypodermal cells are not parallel, 

 but are somewhat inclined toward each other, the outer ends generally 

 converging about centres so as to form groups or packets. The small 

 ones lie out near the free surface of the hypodermis ; and the largest 

 may reach the basement membrane (Fig. 1). Usually the long axes of 

 the rhabditi are approximately perpendicular to the surface of the epi- 

 dermis, but they may assume almost any angle with each other ; small 

 rods are sometimes seen lying at right angles to neighboring ones. 



It was first shown by Oscar Schmidt ('48, p. 6), in 1848, that in the 

 case of Rhabdocoeles the rhabditi are developed in subcutaneous flask- 

 shaped cells. Since that time similar conditions have been discovered 

 in all the Triclads. Up to the present time, the development of these 

 cells, "Stabchenbildungszellen," has not been traced. My studies seem 

 to throw some light on their genesis, and also to show how the rods 

 find their way out between the cells of the epidermis. I first rec- 

 ognized the parent cells in isolation preparations, and saw them in 

 sections only after depigmenting and staining the sections on the slide. 

 Later, I obtained a fresh supply of material, and was able to demonstrate 

 them in abundance, and in all stages of development. They are more 

 easily to be seen on the ventral side of the animal, where they are less 

 obscured by pigment. In their fully developed condition they lie in 

 the body parenchyma immediately beneath the longitudinal muscles. 

 On the ventral side, where the muscle layer is very thick, they may be 

 found in between the strands of the muscles as well as below them. 

 The parent cells have the form of flasks with greatly elongated narrow 

 necks tapering off into long tubular processes, which are traceable out- 

 ward through the muscles to the basement membrane, and, traversing 

 this, are seen to open out between the cells of the hypodermis. Thus 

 the deep-lying parent cells are in direct communication with the outer 

 world (Figs. 1, 6, and 10). It isby means of these tubular processes 

 that the rhabditi find their way to the exterior, and at length come to 

 occupy positions between the hypodermal cells. I have previously 

 pointed out that the rhabditi in the epidermis lie in groups or packets; 

 presumably each of these groups was at one time contained in a single 

 parent cell. 



The connection of the parent cells with the epidermis is a primitive 

 one, for they are only modified cells of the hypodermis, which never 

 cease to retain their connection with that layer. In the earliest stages 

 of development that I have found, they appear like small sacs im- 

 bedded in the superficial portion of the longitudinal muscle band, close 



