2 Mr. A. Alcock on the 



short account o£ three matters of interest in connexion with 

 this specimen, namely : (1) the form of the young embryo 

 itself, (2) the structure and relations of the gill- filaments, 

 which in this stage make up a large part of the bulk of the 

 embryo, and (3) the structure of the maternal nursing-fila- 

 ments, or trophonemata, which at this stage appear to be only 

 preparing for their special secretory function — and, lastly, to 

 offer some suggestions (1) as to the bearing of the facts of 

 the individual history of these embryos upon the problem of 

 the ancestral history of the genus, and (2) as to a possible 

 interpretation through these embryos of the phenomenon of 

 aplacental viviparity among the Elasmobranch fishes. 



2. The Early Embryo o/Pteroplatsea micrura. 



The embryo* now to be described is about 29 millim. long ; 

 it has a remarkable generalized shark-like form (fig. 1), its 

 snout, its gill-openings, and its tail having a Selachoid and 

 not at all a Batoid appearance. 



The snout is produced far beyond the mouth and is bluntly 

 conical. 



The gill- openings, from whatever aspect seen, are remark- 

 able. From the dorsal view the branchial region forms on 

 each side an inflated chamber in which the broad branchial 

 bars are plainly visible ; anteriorly the first slit forms the 

 wide-open spiracle, but the other slits, five in number, are 

 closed, and are conspicuous only because of the large blood- 

 vessels which run in them. From the side view six nearly 

 equidistant clefts are seen, the first being the spiracle and 

 the other five being still closed but very plainly visible on 

 account of their vascularity. It is only ventrally that the 

 gill-slits, here very short and comparatively inconspicuous, 

 are open to give issue to a cloud of delicate filaments, many 

 of which when straightened out are nearly twice the length 

 of the embryo itself, and the sum of which forms at least one 

 third of the whole volume of the embryo. 



The trunk is cylindrical and Selachoid and ends in a thick 

 cylindrical shark-like tail, which bears terminally a long 

 ventral and a shorter but deeper dorsal tail-fold. 



The pectoral Jins are large, their base being coextensive 

 with the length of the trunk ; each is prolonged forward, 

 parallel with but quite separate from the branchial region, 

 and in the same plane with the head, into a tapering bar, 

 which, however subsequently curled, starts with an inward 



* One embryo, typical of all, has been selected for this description. 



