igig-] T. Southwell & B. Prashad : Studies of Indian Fishes. 239 



of the embryo in Pteroplatea micrura} In the more advanced em- 

 bryos, however, the branchial clefts are tightly closed and there 

 are no filaments, those which were formerly present having ap- 

 parently atrophied. These structures, it thus seems, are present 

 only in the earlier stages of the embryonic existence. 



The structures have been called by many different names 

 such as branchial or gill-filaments (Wood Mason and Alcock) ''■ , 

 external gills or gill-filaments (Wood-Mason and Alcock) ^ and 

 trophonematous filaments (Chaudhuri).* The name external gills 

 or gill- filaments suggests that they function as gills and may 

 lead to wrong conclusions being drawn as to their being homolog- 

 ous with or even analogous to the external gills of Amphibia. 

 On the other hand the name trophonematous filaments would 

 lead one to think that they were structures for the absorption of 

 nutriment. But as the name trophonemata has been used by 

 Wood-Mason and Alcock for the '' narrow, strap-shaped nourishing 

 processes " of the uterine wall of Batoids. we do not think it 

 desirable that the same or an essentially similar name should be 

 given to processes of the embryo. In our opinion the name 

 branchial filaments is the most suited, as, besides showing their 

 origin, it does not suggest or imply any function for these struc- 

 tures. 



The branchial filaments have been already described in the 

 descriptions of the various embryonic forms , their histological 

 structure has been admirably treated by Alcock in his description 

 of the " Embryonic History of Pteroplatea micrura " {Ann. Mag. 

 Nat. Hist., Vol. X, pp. 3, 4, 1892), and we have nothing to add to 

 that account. 



As to their function, Alcock in the paper cited above con- 

 sidered them to be of use for absorbing the nutriment in the yolk- 

 sac of the embryo. Their very elaborate vascular supply, on the 

 other hand, points to their being of the nature of respiratory 

 structures, possibly in addition to their being of use in the absorp- 

 tion of yolk and the free secretions of the maternal uterine wall. 



Summary. 



In the general observations we have described certain struc- 

 tures developed by Elasmobranch embryos during different periods 

 of their intra-uterine existence. These structures result in very 

 definite changes in the modes of obtaining nourishment, and may 

 be summed up as follows : — 



I. In the placental forms, in the earlier embryonic stages, 

 there is no placenta and the yolk-sac functions as such. Later on, 



t Alcock, Ann. Mag. Nat. Hist. (6), X, p. 2 (1892). The correct name of 

 the fish according to Garmaii as stated previously is P. poecilnra. 

 2 Pyoc. Roy. Soc, XIAX, p. 363 (1891). 

 ** Proc. Roy. Soc, L, p. 204 (1891). 

 * Mem.Ind. Miis., V, p. 409 (1916). 



