THE HYDRA 89 



cover glass without removing the stain. Before discarding any prepa- 

 ration make sure you have made tlie most of it. 



(g) Separate your sheet of drawing paper into two areas by means 

 of a light line. ]\Iark one region for "Cells of the Ectoderm" and the 

 other for ''Cells of the Endoderm." As you find and identify the sepa- 

 rated cells, draw them (5 X measured size as projected to table level) 

 in the correct region ; indicate functions of cells in your labels. The fol- 

 lowing types of cells will be recognized in favorable preparations: (1) 

 Endoderm cells, the largest ones present, usually elongated, with a 

 conspicuous vacuole or vacuoles in the cytoplasm, and with muscle 

 processes extending from the base. If green hydras are used, the endo- 

 derm cells will contain numerous green, unicellular plant cells, or 

 Zoochlorellse. (2) Gland cells, elongated, slender, and frequently 

 tapering toward one end, with deeply stained granular cytoplasm. 

 (3) Ectoderm cells, similar in shape but smaller than the endoderm 

 cells and bearing muscle processes which will usually be contracted. 

 Why? (4) Interstitial cells, very small, rounded cells, with relatively 

 large nuclei. (5) Cnidoblasts, or stinging cells, containing undis- 

 charged nematocysts. Recognize the nucleus, cytosome, and cnidocil. 

 Find as many types as possible of discharged nematocysts. (6) Nerve 

 cells from the nerve net are sometimes seen. 



Exercise 5. — Regeneration and Reproduction. 



(h) The phenomenon of regeneration is widespread but is most 

 highly developed among those groups of animals which reproduce 

 extensively by asexual processes such as budding and fission. Clean 

 two watch glasses, filling one of them two-thirds full of water from a 

 jar in which hydras have been living, and using the other as a cover. 

 Take several hydras and cut each transversely into two or more pieces. 

 Examine the pieces with the low-power objective of the compound 

 microscope. Set aside and examine at subsequent laboratory periods 

 until the regeneration is complete. Make sketches and briefly record 

 the changes of the several pieces. Compare with the well-known 

 powers of regeneration and vegetative reproduction in plants. 



(i) Hydras reproduce by budding throughout the period when 

 seasonal and food conditions are favorable. Budding is essentially 

 reproduction by cell division, an asexual method. Observe and draw 

 three progressive stages. Understand the relations between the layers 

 of the parent and bud. 



(j) Hydras reproduce by syngamy during certain periods. Gonads 

 are formed in the ectoderm. The ovary contains a single ovum, which 

 is exposed to the water when meiosis has occurred. The testis, of 



