260 W. R. Coe — Development of the 



Plate XXXIV. 



The mesodermic structures are prioted in red. 



Figure 1. — Optical, median, sagittal section of the early pilidium of C. leidyi, thirty- 

 six hours after fertilization. To the section the right lappet has been added, and 

 likewise the right half of the apical depression. The nuclei and cell-boundaries 

 have been drawn from an actual section. The mesoderm cells are arranging 

 themselves in their definite positions, x 375. 

 Figure 2. — Optical, transverse section of the pilidium of J/, cceca at the age of ten 

 days. The section passes through the mouth, esophagus and apical plate (of. 

 fig. 3). Below the esophagus are sections of the two horizontal lips (Z) with 

 strong cilia bordering the mouth. The lappets are contracted, and therefore 

 thicker than in life. Just inside the epithelium (both of the ectoderm and ento- 

 derm) are drawn in red the cells and fibers of the larval musculature. The 

 larger, dotted, red cells are those of the mesenchyme. The single, coarse flagel- 

 lum is seen to be made up by the consolidation of several more slender flagella. 

 The mesoderm cells are grouped mostly in the lappets, and just beneath the 

 apical plate, x 500. 



Figure 3. — The left half of a pilidium of Jf. cceca of ten days. A median, sagittal 

 section is represented in stipple. In addition to this the left buccal ridge (a) is 

 shown in surface view at a lower level. This buccal ridge is seen to be continu- 

 ous with the left lip at the border of the mouth. Below the lip the short lappet 

 is shown somewhat contracted. The mouth is widely opened and extends hori- 

 zontally between the lines converging at m. The intestinal sphincter is partially 

 contracted. The mesodermic structures are indicated as in fig. 2. Two mesen- 

 chyme cells with amoeboid processes lie above the digestive canal, and several 

 others without processes lie beneath the intestine, x 500. 



Figure 4. — Transverse section of a lappet more highly magnified to show the relation 

 of the mesodermic structures to the ectoderm -cells. Three muscle-cells are 

 shown with their fibrous processes inserted between the epithelial cells. Near 

 the angle of the lappet a single, rounded mesenchyme cell is represented. The 

 letter b indicates the ectoderm of the lateral surface of the body ; c, that of the 

 under surface. 



Figure 5. — Five partially developed muscle-cells with contracted processes after kill- 

 ing with corrosive sublimate. 



Plate XXXV. 



The mesoderip is printed in red. 



Figure 1. — Median, sagittal section of the pilidium of M. cceca at the end of the third, 

 day. This is a much earlier stage than that shown in fig. 3, PI. xxxi. The 

 muscular cells have attached themselves in their definite positions, and have 

 already begun to send out fibrous contractile processes. Not all of the mesen- 

 chyme-cells are distinguishable from the muscle-cells. The outline of the lappet 

 is indicated beneath the section. One of the buccal ridges (a) is indicated, as is 

 also the extent of the mouth (m). x 500. 



