LIFE HISTORY OF DESMOGNATHUS FUSCA. 2QI 



along the dorso-lateral regions; it proceeds so rapidly during the 

 few succeeding days that in stage D the process is almost com- 

 pleted. 



Two sets of highly specialized integumental organs have 

 already made their appearance at the time of hatching. One 

 of these consists of the neuromasts, or sense organs (sri) of the 

 lateral line type, which are abundantly distributed over the head, 

 especially the snout, and in two lines which extend along each 

 side of the trunk, while a single line upon each side extends along 

 the tail. These organs, each of which consists of a group of 

 specialized epithelial cells, begin their development long before 

 hatching and at the time of hatching appear to be fully formed 

 so far as the number of cells entering into them is concerned. 

 The surrounding accessory cells probably undergo some further 

 modification. 



The other organs to be mentioned are multicellular glands of 

 the alveolar type. They are limited in number and occur in 

 definite locations; a small group of two or three posterior to each 

 eye (the post-orbital group, po, Fig. 14, a), a group of from one 

 to three over the otic region (the supra-otic group), a larger 

 group of from six to eight on each side of the head posterior to 

 the otic region and dorsal to the gill bushes (the supra-branchial 

 group, sbr, Fig. 14, a), and a single row approximately segmen- 

 tally arranged along the middorsal line of the trunk (ds, Fig. 14, b) , 

 becoming, in the transition to the caudal region, a double row 

 in slightly alternating, segmental pairs (cd, Fig. 14, c} lying 

 along the line of junction of the dorsal finfold and the tail. 

 With the exception of those at the posterior end of the caudal 

 series and a few of the supra-branchial group, these glands are 

 very large, measuring in diameter from two to eight times the 

 thickness of the epidermis; and although they have developed 

 from the epidermis they lie quite beneath both the epidermis and 

 corium within the loose layer of subcutaneous tissue, and open 

 to the exterior by a slender duct which leads through the skin 

 (Fig. 15,0). Some of them have already made their appearance 

 in the i8-day embryo as a mass of rapidly dividing epithelial 

 cells, and in the 13 mm. embryo of 30 days' development are 



