522 STANLEY C. BALL 



while those of the neighboring tissues were entirely without 

 mitotic figures (fig. 49). 



Just previous to this stage of development occasional large 

 cells make their appearance in the ventral region of the body 

 between the entoderm and the layer of muscle fibers. These 

 cells are represented in figures 37 and 43 as having larger, clear 

 nuclei and a considerable amount of darkly stained cytoplasm. 

 The color of the latter in iron-haematoxylin preparations is a 

 bluish gray. 



It is believed that these large cells later develop into the yolk 

 cells of the vitellaria. The grounds for this conclusion will be 

 brought forward in the section on post-embryonic development. 

 But their behavior while the young worm is still enclosed in the 

 capsule may be traced somewhat farther. Since they appear 

 first in the region close behind the sex organ anlagen it is probable 

 that they arise, either directly from them or from cells which 

 have had a similar ancestry. It is known of other Turbellaria 

 that the vitellaria become differentiated from the same mass' 

 with the ovaries and the testes (Hallez '09). So in Paravortex 

 gemellipara, which at the height of its reproductive activity is 

 characterized by the presence of remarkably extensive vitelline 

 glands, the general rule holds concerning their origin; but in 

 point of time their branching off from the common sex organ 

 mass seems unusual. 



/. Cilia. The epithelial cells become furnished with cilia shortly 

 after the stage seen in figure 34. They are already present in 

 figure 35, but of their development nothing has been observed 

 beyond the fact that, although they are as long when first noted 

 as in older worms, they are not nearly so numerous as on the 

 adult epithelium. The embryos at this stage are able to move 

 about in the capsules. The ciliary action can easily be observed 

 through the maternal tissues when the living worm is studied. 



g. Embryonic digestion and early development of the intestine. In 

 an earlier section of the paper it was explained that the gastru- 

 lating embryo, like those of all Rhabdocoeles, enclosed within 

 itself the free yolk material which had previously entered the 

 capsule as a constituent of the vitelline cells. The greater 



