OBSERVATIONS ON THE DEVELOPMENT OF RAIA IUTIS. J ft 



5. There are no temporary branchial fringes or filaments on (lie first and seventh 

 arches; on the others the fringes are developed from the outer and convex portion of 

 the arch, and are not at first prolongations of the internal gills. 



6. The nostrils, as in all vertebrates, consist at first of pits or indentations in the 



integuments; secondly, a lobe is developed on the inner border of each ; and finally, 

 the two lobes become connected, and thus form the homologue of the fronto-nasal pro- 

 tuberance. The transitional stages of these correspond with the adult conditions of 

 them in other species of Selachians. 



The nasal grooves are compared with the nasal passages of air-breathing animals, 

 and the cartilages on either side of these to the maxillary and intermaxillary bones. 



8. The foremost part of the head is formed by the extension of the facial disk for- 



extension is going on, the cerebral lobes change their position from 



beneath the optic lobes to one in front of them. 



9. Two anal fins, one quite large and the other very small, are developed, but both 

 are afterwards wholly absorbed. 



10. The dorsals change position from the middle to the end of the tail. At the time 

 of hatching, however, there is still a slender terminal portion of the tail, which is 

 afterwards either absorbed or covered up by the enlarged dorsals, as they extend 

 backward. 



ard : while this 



EXPLANATION OF THE FIGURES. 



Fig. 1. Egg case, one half of the natural size, linear measurement ; a, pouch 



and outward flow of water; c, spur ; d, membrane formed by the interweaving f the lateral 



filaments. 

 Fig. 2. 



iphal 



anal 



Fig. 3. Under side of the yelk of the preceding specimen ; a, the continuation of the artery seen in fig. 2, 

 and connecting with the triangular terminal sinus. 



Fig. 4. A more advanced embryo, showing at a and b the pectorals and ventrals ; d, the temporary anal. 



Fig. 4*. Head of the preceding enlarged ; b, first branchial arch, without fringes ; d, projection of the 

 optic lobes ; e, projection of the cerebral lobes ; the open space between the first branchial arches is the mouth. 



Fig. 5. Side view of the same ; a, first branchial fissure, largest at its outer end ; this enlarged portion 

 corresponds with the future spiracle ; 5, the inner end ; the first arch is in front of this fissure ; £', the second 

 fissure, in front of which is the second arch, bearing a fringe ; c, nasal fossa ; d, projection of the optic lobes ; 

 e, cerebral lobes. 



Fig. 6. A shark-shaped embryo ; c, dorsals ; d d' f anals. In this figure the embryo is represented as 

 twisted on the yelk, through half a circle, consequently the artery is directed backward instead of forward. 



