Ayers, Nasal rays in Condylura cristata. 359 



for three millimetres toward the base of the snout and is marked off 

 from the reraaiiing- surface by a series of furrows ninning- parallel to 

 tlie long axis of tlie body. A series of parallel ridges is tlms formed, 

 each ridge being boundcd on either side by a fuiTow. At their an- 

 terior and posterior ends these ridges pass gradnally into the neigh- 

 bonriug smooth surface. By a gradnal iugrowth of the bottoms of 

 the furrows each groove is deepened and each ridge suffers a corre- 

 spondingly increased definition of form, while at the same time the 

 posterior end of each groove grows toward its neighbour on either 

 side. Wheu the grooves have all united, there is formed by their 

 Union a common groove which nearly encircles the snout and sepa- 

 rates the tactile from the remaining surface of that organ. 



Commencing at the posterior margin of the tactile surface and 

 advancing toward the tip of the snout, the grooves deepen and grow 

 toward each other in their bottom portious until they finally coalesce 

 underneath the ridges. The result of this process is the production 

 offree, finger-shaped processes composed exclusively of ectoderm, at- 

 tached to the anterior end of the snout in the manner already de- 

 scribed for the adult. 



These processes of the ectoderm become the tactile rays of the 

 adult. The nasal area from which the tentacular processes are formed, 

 is not thereby deuuded of skin, but remains covered by that portion 

 of the primary surface which formed the bottoms of the grooves and 

 which has so increased in extent, that at this stage the surface is 

 entirely and uniformly covered by ectoderm. No traces are left either 

 on the surface or in the corium of the extensive excision which have 

 taken place. 



The principal details of the process are readily scen on examin- 

 ing a section of the snout, such as is represented in Fig. 3. In this 

 figure I have drawn, with the aid of camera-outlines and with dia- 

 grammatic shading, a transverse section of that part of the nose of 

 a young Condylura indicated by 'the line r. Fig. 1. The following 

 is a Short account of the most importaut histological details of the 

 process. The entire circumference of the section is boundcd by a 

 thiu layer of epidermal cells e, beneath which all the formative pro- 

 cesses take place. In the stage of development represented in 

 Fig. 3. this layer only loosely Covers the snout in the region of the 

 papillae, and later is entirely cast off; but it remains in intimate con- 

 nection with the remaining surface and functions as the true epidermal 

 layer, as at /. The letters o. p. designate respectively the epidermis 

 of the tentaculiferous area and that of the sense rays. The rays are 

 imbedded in a layer of fibrous tissue which however does not entirely 

 Cover the outer surface of the ray. 



Sections of the ray present a crenate margin owing to their pas- 

 sage through the numerous tactile papillae which cover the surface 



