520 STANLEY C. BALL 



d. Musculature. Like the nervous system and the pharynx 

 the mesenchyme and dorsoventral muscle cells arise from the 

 undifferentiated mass which occupies the anterior portion of the 

 embryo at the close of gastrulation. If one compares figure 

 23 and figure 24 it is noticed that in the latter indifferent cells 

 have migrated posteriorly so that a continuous sheet of them hes 

 between the ectoderm and entoderm. Their amoeboid nature 

 is further evidenced by the extreme delicacy of their membranes. 

 Indeed, it is impossible to distinguish a boundary where two 

 cells come in contact with each other. The tissue at this stage 

 may be called a syncytium. Occasional mitoses are observable 

 in this region. The plane of the section of the embryo at the 

 right in figure 24 is sagittal, and somewhat toward the left side 

 of the body, and the dorsal margin lies toward the top of the 

 page. Thus it appears that in P. gemellipara the migration of 

 cells occurs as freely in the dorsal as in the lateral and ventral 

 portions of the embryo. In the later stages, however, as Hallez 

 pointed out, there are comparatively few nuclei on the dorsal 

 side of the entoderm (fig. 37). 



There are two types of nuclei in this sub-epithelial layer, an 

 external row of small rather deeply stained nuclei which usually 

 lie against the inner surface of the ectoderm cells, and an internal 

 series of larger and paler nuclei, some of which are slightly irreg- 

 ular. The second type is more numerous on the lateral and 

 ventral surfaces. The embryo at the left was tangentially 

 sectioned, thus showing the more lateral nuclei. 



Passing to the more advanced stage shown in the sagittal 

 section in figure 37, where the brain and pharjoix have become 

 differentiated, it is found that certain of the cells containing the 

 smaller nuclei have become spindle-shaped. The cytoplasm 

 has formed a process at each end. These spindle cells are still 

 more conspicuous in figures 27 and 36. 



Bresslau ('04) describes similar cells at a corresponding period 

 in the development of Mesostomum as representing the early 

 stages in the differentiation of the dorso-ventral muscle fibers. 

 The number observed just beneath the ectoderm of Paravortex 

 rather suggests that from these cells arise the circular and longi- 



