24 ROYAL SOCIETY OF CANADA 



embryo. Their endothelial tubes remain independent throughout, but 

 the splanchnic mesoderm ^ does not at first dip in very far dorsad so 

 as to furnish an independent wall for each heart. Further forward, 

 however, it does so (Fig. S), and eventually the two bulbs of the 

 heart are widely separated and enclose between them a portion of the 

 common cœlome (Fig. 7). But the two heart-tubes as seen in Fig. 9 

 do not contract gradually into the condition seen in Fig. 8; on the 

 contrary, there is a marlced constriction at the opening of each heart 

 into its buPous, beyond which a ventricular cul-de-sac extends cephalad 

 for a few sections on each side. 



The picture presented by Fig. 6 is best calculated to show the 

 anterior duplicity of the vascular system, because when each bulbus 

 approaches the stomatodaoum it divides into two ventral aortee. Of 

 these the lateral aortas alone form arches up the sides of the pharynx, 

 for the median ones first anastomose below the pharymx, then subdivide 

 into four small vessels which bend round its .anterior surface, and 

 f iially open into the large vascular space represented in Fig. 5, situ- 

 rted between its anterior diverticula. Tracing this space backwards 

 dorsad of the composite pharynx, we first find four vessels similar to 

 tliose referred to al)ove, which soon, however, fuse into the " median " 

 dorsal aorta. Tliis retains its size until we reach the segmented region 

 of the embryo, in which it tends to be obliterated opposite the somites 

 and to expand again intersomitically. The "■'lateral'" dorsal anrtii^ con- 

 duct themselves as in a normal embryo, and the same may be said of 

 the veins as far as they are developed. 



EXTODERMTC TkACT. 



Proceeding cephalad from Fig. 31 in which the median ridge 

 formed of the median row of somites alone distinguishes this from 

 the entoderm of a normal embryo we find nothing remarkable until 

 about midway between Figs. 8 and 0, there the lateral pouches of the 

 pharynx reach a little nearer the ectoderm in the region of the first 

 gill-clefts, but a few sections further forward (Figs. G and 5) the two 

 stamatodœa at once arrest attention, as do the two anterior diverticula 

 corresponding to the pouches of Seesel of normal embryos. 



I venture to enter a mild protest against Professor Kaestner's note (No. 

 6, p. 128) on the usage of the words somatopleure and spanchnopleure. 

 Surely, if it is desirable to have mononyms for " somatic mesoblast," and 

 " splanchnic mesoblast," it would be easy enough to form them instead ot 

 using terms which were invented and are constantly used to designate 

 something else. If the language of anatomists knows only one meaning for 

 nXevpa' that of zoologists is not so restricted. A Pleuronectid does not swim on its 

 " pleura ! " 



