Embryonic History o/" Pteroplatcea micrura. 3 



twist. If the head were more depressed, and if at the same 

 time the prolongation of the pectoral fins were broader and 

 more truncated, the result would be a by no means fanciful 

 resemblance to Rhina. 



The ventral Jins are small and lie well free from the hinder 

 limit of the pectorals. 



The embrjo is still attached by a broad cord, about 

 12 millim. long, springing from the belly between the front 

 border of the pectoral tins, to a large yolk-sac, which appears 

 to consist entirely of a diffluent yolk hardly more stable than 

 oil, enclosed in a membrane of extreme tenuity. The gill- 

 filaments are in intimate relation with this yolk, closely and 

 completely enveloping it on all sides. The cord of attach- 

 ment is so delicate, yet so broad withal, that I have not 

 succeeded in cutting complete transverse sections ; but this 

 much is quite certain, both from examination of partial trans- 

 verse sections and from examination of portions of a stained 

 cord mounted flat as transparent objects in glycerine, that it 

 consists of a solid mass of close-packed large-nucleated cells, 

 and is longitudinally traversed by numerous lacuniform 

 channels of very irregular outline and of unequal size, and 

 that these channels contain NOT BLOOD- CELLS, BUT SMALL 

 SPHERULES OF YOLK ONLY. In short, nothing of the nature 

 of a distinctly defined artery or vein, or indeed of any vessel 

 containing blood-cells, is to be made out ; and this must be 

 looked upon as a fact of the highest significance, not merely 

 when we come to seek an explanation of the use of the gill- 

 filaments in this species, but when we come to consider the 

 much wider question of embryonic nutrition among the 

 aplacentally viviparous Elasmobranchs in general. 



3. The Gill-filaments of the Early Embryo of 

 PteroplatEea micrura. 



These issue ventrally from all the gill-slits except the 

 spiracle, and closely embrace the yolk-sac ; if they have any 

 additional attachment to the uterine wall it must be of the 

 feeblest nature. Their total volume in the fresh state was 

 not less than one third that of the entire embryo. 



They vary in length, most of them being considerably 

 longer than the embryo itself; their breadth is about 0*5 

 millim. and their thickness is quite inappreciable by the 

 unaided eye. 



A filament stained with cai mine, mounted flat in glycerine, 

 and examined as a transparent object under a low power 

 (fig. 2) has a uniformly granular appearance — due to the 



1* 



