26 LABORATORY DIRECTIONS IN 



layer, or are all approximately alike? How do the cells of the endoderm 

 differ from those of the ectoderm? What are the large clear spaces in 

 the endoderm cells? 



Draw a small portion of a section, showing all the different kinds of 

 cells you have studied. Choose for drawing a portion of a section where 

 the cells are as diagrammatically arranged as is possible to find. The 

 figure should represent the thickness of the two layers as about two inches. 



55. Germ Cells. If available examine a living specimen bearing one or 

 more spermaries or testes. What is the shape of this organ? Do you note 

 any movement within the spermary? The moving bodies are the sperma- 

 tozoa. Sketch an entire specimen showing the spermaries. (Use a pre- 

 pared slide if a live specimen is not at hand.) Examine a cross-section of 

 Hydra through a spermary. The spermatozoa are deeply stained cells 

 in a dense mass. What is their relation to the ectoderm and endoderm? 



Examine either a living or a stained specimen bearing an ovary. 

 Sketch to show this organ. What is the relation of the ovary to the ecto- 

 derm and endoderm? (When an ovum is fertilized by a spermatozoon, 

 it develops into an embryo. See demonstration.) 



Are the somatic cells of Hydra all alike? If not, how many kinds 

 may be observed? Are the cells of one kind grouped together, or scat- 

 tered over the body? If the answer to the last question is different for 

 different kinds of cell, specify the difference in your notes. Are the germ 

 cells all alike? 



Do you observe any advance in complexity of Hydra over Volvox? 



6. The Earthworm (Lumbricus terrestris). Study both living and 

 preserved specimens. 



6a. External Features. Note that the body is divided into segments 

 known as somites or metameres. A segmented animal is said to be 

 metameric, or to exhibit metamerism. ' 



Observe that the animal has a dorsal or upper surface, and a ventral 

 or lower surface. It has also an anterior end and a posterior end. Con- 

 sequently it has also a right and left side. Since the earthworm can be 

 divided by only one plane into two corresponding halves, it is said to be 

 bilaterally symmetrical. Where does this one plane pass? 



The following external features are referred to in the dissection : , 



Seta, minute horny bristles arranged in rows on each side of the 

 body. Pass a preserved worm through your fingers in both directions. 

 What does the result indicate? 



Clitellum, a swelling of the body in the region of metamere 32. On 

 its ventral side is a pair of thickened ridges, the tubercula pubertalis. 



Prostomium, a small rounded projection at the anterior end, overhang- 

 ing the mouth. 



Mouth, an opening at the anterior end leading to the buccal or mouth 

 cavity. 



