90 EDWARD PHELPS ALLTS, JITN. 



epidermal layer of the ectoderm, and then the entire ecto- 

 derm, thus first exposing" the tubules and then the canals ; 

 and finally to ^^skin" a head in a single piece, and examine 

 it in glycerine by transmitted light. Fig. 7 is made from 

 such a preparation. 



Lateral and Ampullary Sensory Systems. 



The manner in which the lateral canals develop in sela- 

 chians is evidently quite different, in certain respects, from 

 that that pertains in ganoids and teleosts. What it is I can- 

 not make out from my three series of: sections, and I do not 

 find in the works at my disposal any description of it that 

 seems complete or satisfactory. 



Balfour (8) leads one to suppose that there is in selachians 

 simply an abbreviation of the process that gives origin to the 

 canals in ganoids and teleosts. This abbreviation consists in 

 that it is the inner row of cells (Schleimschicht) alone of the 

 epiblast that is concerned in the involution that forms the 

 canal, the epidermis remaining always as a flat and even 

 layer above it. The canal, thus formed, then becomes de- 

 tached from, and sinks beneath, the epiblast, remaining 

 attached to it at certain points by cords of tissue which 

 represent the primaiy tubes of the canal. Opposite the outer 

 ends of these primary tubes the epidermis then becomes 

 perforated, thus giving rise to the primary pores. 



The ampullary tubules are said by Balfour to arise in 

 exactly the same manner as the lateral canals. 



Balfour says that the lateral canal of the body, developed 

 as above set forth, first appears at the hind end of the lateral 

 sensory line, and extends forward from there. Mitrophanow 

 (44, pp. 208-9) confirms the latter part of this statement, 

 but the involution of the canal as he describes it seems to 

 involve the entire ectoderm, and not simply its deeper, inner 

 layer. Moreover, it would seem from his figures as if the 

 section of canal said to be thus first formed was nothing more 

 nor less than what Clapp (10, p. 239) refers to as "a curious 



