Pilidiimx of Certain Nemerteans. 259 



Figure 3. — A gastrula of M. ccEca, seventeen hours after fertilizaticn. The dark ring 

 in the center indicates the blastopore. The cell-boundaries and the nuclei are 

 drawn as accurately as possible from an egg fixed in picro-acetic, stained with 

 acidulated hfematoxylin and mounted in oil of cloves. The anterior surface is 

 towards the top of the page. Cilia are not indicated, x 600. 



Figure 4. — Vertical section of the gastrula of C. leidyi, seventeen hours after fertiliza- 

 tion. The apical plate is indicated by darker stipple. The mesoderm-cells lie at 

 the sides of the eutodermic invagination. The segmentation cavity is smaller 

 than normal owing to the shrinkage which took place during the process of im- 

 bedding in paraffin. One of the endoderm cells is dividing vertically, x 500. 



Figure 5. — A pilidium of C. leidyi at the age of ten days. The dotted lines indicate 

 the position and thickness of the walls of the enteric cavity ; o, cavity of esopha- 

 gus ; z, cavity of intestine, x 250. 



Figure 6. — Pilidium of J/, cceca at the eighth day. The nuclei on the surface are those 

 of the ectoderm cells, the boundaries of which are indicated by faint, dotted 

 lines The coarse, dotted line indicates the position of the enteron, as in fig. 5. 

 The mouth is indicated by the heaviest dotted line (cf. PL xxxiv, fig. 3). Below 

 the mouth is seen one of the very short lappets with its strong cilia and numer- 

 ous nuclei, x 400. 



Plate XXXIII. 

 In all the figures the cells of the larval mesoderm are indicated in red. 



Figure 1. — Median, optical, sagittal section of gastrula of M. cceca fifteen hours after 

 fertilization. The cells of the apical plate with their developing fiagella are in- 

 dicated by the darker stipple. The cilia, which cover the whole surface, are not 

 shown. X 600. Corrosive sublimate, acidulated hgematoxylin. 



Figure 2. — A similar section of the very early pilidium of M. cceca twenty-four hours 

 after fertihzation. Below the section is a line which indicates the extent to 

 which the lappets have developed. The boundaries of the cells are merely indi- 

 cated. Slightly diagrammatic, x 600. 



Figure 3. — Median vertical section of blastula of C. marginalus. At the right of the 

 elongated entoderm cells is a larger cell, m, in process of division. This cell 

 appears to be homologous with one of the primary mesoblast cells of the annelid 

 embryo. Immediately to the left of m are two minute cells which bear a strik- 

 mg resemblance to the annelid "posterior entoblast" cells. Picro-acetic. x 350. 



Figure 4. — Early gastrula of C. marginalus. The cells of the apical plate are indi- 

 cated by darker stipple. The blastopore is very narrow. The entoderm cells 

 have not yet become arranged to form the intestinal canal. Picro-acetic. x 300. 



Figure 5. — Median, sagittal section of late gastrula of C. marginatus. Picro-acetic. 

 x350. 



Figure 6. — Median, sagittal section of the embryo of C. marginatus at the beginning 

 of the pilidium-stage. The extent of the lappets is indicated by the line (with 

 cilia) below the section. On the dorsal side the section must be considered as 

 being of great thickness, for the drawing represents not only a section of the 

 apical plate, but also a surface view of half of the depression above it from which 

 the flagellum issues. The two darker cells in the intestinal wall are gland-cells. 

 The intestinal sphincter (devoid of cilia) is partially contracted. Sligbtlj'^ dia- 

 grammatic. Picro-acetic. x 350. 



