LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



The dogfish is covered with placoid scales, which are much smaller and more 

 closely set together than in the case of the skate. The spines of the scales can 

 be felt by passing the hand over the dogfish skin. By boiling a piece of the skin 

 with alkali, the scales may be separated out. Examine such isolated scales 

 under the microscope. They are similar to those of the skate, with rhomboid 

 basal plates and a projecting spine. 



2. The homology of teeth and placoid scales. The most interesting point 

 about the placoid scale is that its structure and mode of origin are exactly the 



stratum 

 germinativum 



derm is 



FIG. 12. Five successive stages in the development of the placoid scale of the dogfish. A , gather- 

 ing of the cells of the dermis to form the dermal papilla. B, evagination of the dermal papilla and 

 secretion of the dentine (colored black) by the outer cells of the papilla. C, continued secretion of dentine, 

 thinning of the interior of the papilla to form the pulp. D, beginning secretion of the basal plate and 

 formation of the enamel (left white) by the under surface of the stratum germinativum. E, eruption 

 of the spine through the epidermis, completion of the basal plate and pulp. (From Goodrich in Part IX 

 of Lankester's Treatise on Zoology, courtesy of the Macmillan Company.) 



same as those of the teeth of all vertebrates. From this it follows that teeth 

 and placoid scales are homologous structures, and teeth are merely slightly modi- 

 fied placoid scales. The homology is probably due to the fact that the lining of 

 the mouth cavity is really skin turned in, and hence may be expected to give 

 rise to structures similar to those found in the skin. 



Obtain a longitudinal half of an ordinary vertebrate tooth. Identify the 

 following parts : the crown or shiny upper part, corresponding to the spine of the 



