A Sycon Sponge. II 



pavement epithelium, ectoderm, quite different from the 

 lining cells of the radial tubes. 1 The incurrent pores through 

 which the sea-water enters the radial tubes are small and 

 somewhat difficult to demonstrate. They may frequently 

 be found in section, as interruptions of the even course of 

 the endodermal lining along the peripheral portion of the 

 radial tubes. They are more easily determined as minute 

 openings in such portions of the section as permit the exam- 

 ination of the surface of the entoderm. 



Make a drawing of the cross-section just examined. 



Reproduction. In the stained specimens there will fre- 

 quently be found large amoeboid cells in the substance of 

 the mesoderm. Such cells, as was the case in the Ascon, 

 are the reproductive cells, male or female, and in a single 

 sponge may occur in varying stages of development. The 

 sperm-spheres and the segmenting ova are clearly covered 

 by a layer of epithelial cells. The sexes are usually not 

 united in the same sponge-individual. 



Draw one of the reproductive cells and show its nucleus 

 and nucleolus. Draw also a sperm-sphere or a segmenting 

 ovum, showing its position in the mesoderm and its epithelial 

 covering. 



1 The difference in the lining of incurrent and radial canals is clearly 

 seen in specimens that have been cut at right angles to the axes of 

 these canals. While the endodermal tubes appear as circles of broad 

 outline, the ectoderm of the incurrent canals forms a most dt'licate 

 circle. 



