ATION 



DEVELOrM ST OF RAIA B ATIS. 37 



In advanced embryos there is formed behind the mouth a semicircular fold of skin, 

 which extends from one angle to the other, and very closely resembles the lower Up 

 of mammals in its first stage of development. AVe did not find this in an adult speci- 

 men which was especially examined for comparison. 



Nostrils. — In the adult, each nostril is lodged in an inverted cartilaginous c\ 



b D 



connected with the base of the cranium, and lined with a folded scnsitii 



cup 

 mucous 



membrane. From this there extends to the angle of the mouth a deep p-oovc, which 

 ends just in front of the « upper jaw." The integument between the right and left 

 grooves projects from the general surface, forming a sort of upper lip (fig. 11, rf), and 

 the angles are developed into fringed lobes which cover the corners of (lie mouth. 

 Each lobe contains a cartilage, which Muller compares with the cartilages of the wings 

 of the nose, as also the outer border of each groove opposite to them. The peculiar 

 configuration of these parts in the adult skate, and the resemblance of the whole to an 

 embryonic condition of the higher vertebrates, renders the study of them quite impor- 



tant. As Professor Ag 



fail to be impressed with this resem 



blance who compares the head of an embryo quadruped, looking at it in front face, with 

 the adult skate."* To give demonstrative evidence that the parts thus compared are 

 homologous, can only be done by an examination of a larger number of specimens, in 

 different stages of development, than have as yet been studied. The series here 

 described contributes something to this end, and enables us to determine some points 

 which have not thus far been noticed. 



The first traces of the olfactory fossae which we have seen consisted of two small, but 

 elongated and well-defined pits (fig. 5, c), somewhat enlarged at each end, and con- 

 verging towards each other backward. They are at a distance from the mouth, and 

 have no connection whatever with it, nor is there as yet the beginning of the groove 

 which is found at a later period. In these respects they resemble the primary form 

 of the nostrils of vertebrates in general. The position of them is such that they might 

 be easily overlooked, for they are confined mostly to the hinder face of that portion of 

 the head which is formed by the projection of the cerebral lobes downward, and can 

 only be wholly seen by standing the embryo on its head. As the mouth takes its 

 permanent shape, the nostrils lengthen, and a process forms on the inner border of each 

 of them (fig. 10, a), which is the first stage of the lobe already described as existing 

 in the adult. By a gradual thickening of the integuments, these processes become con- 

 nected with each other across the middle line, when the whole skin between the nos- 



■tory, (B 



