78 EMBEYOLOGY OF THE LOWEK VEETEBEATES ch. 



as they occur in a South American tadpole {Paludicola) 1 but the 

 description fits quite well the mode of development as it occurs m 



Tadpoles generally (Keiffer, 

 Gutzeit),the differences between 

 different species and genera, 

 though of systematic import- 

 ance, being differences in detail 

 such as shape and arrangement 

 of the individual teeth of the 

 comb. 



" Teeth " of Cyclostomes. — 

 The horny teeth of cyclosto- 

 matous fishes, though they 

 would naturally fall to be 

 treated in the next chapter, 

 situated as they are within 

 the buccal cavity, may con- 

 veniently be considered now 

 owing to their resemblance — 

 on a much larger scale and with 

 multicellular structure — to the 

 horny denticles of the tadpole. 

 The tooth-like spines of the 

 cyclostome are cones of highly 

 cornified epidermal cells. Each 

 tooth develops in the substance 



of the epidermis (Fig. 46, A) 

 being strikingly like a hair- 

 rudiment during early stages. 

 Successional spines develop be- 

 neath the bases of the func- 

 tional ones as shown in Fig. 46. 

 Glandular Developments 

 of the Epidermis. — In the 

 Anamnia it is usually the case 

 that scattered cells of the epi- 

 dermis take on a glandular 

 function and serve to form 

 a slimy secretion which amongst 

 other functions serves to dimin- 

 ish the " skin-friction " which is 

 the main resistance to movement 

 through water. Such unicellular 



Fig. 47. — Illustrating the development of the 

 cement - organ of Lepidosiren as seen in 

 sagittal sections. 



A, stage 23 ; B, stage 25 ; C, stage 31 ; D, stage 35. 

 In A the rudiment of the cement-organ is seen to be a 

 thickening of the deep layer of the ectoderm ; in B 

 and C the superficial layer has disappeared over the 

 thickened glandular area ; in D the organ is commen- 

 cing to shrivel and crowds of phagocytes are collected 

 in its neighbourhood. 



glands may become collected 



together to form multicellular glands. Of these the most conspicuous 



examples are found, outside the Mammalia, in the Lung-fishes and 



Amphibians — where they form the flask-glands and the cement-organs, 



1 Probably P. fuscomaculata according to Boulenger. 



