542 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [88] 



" The early formation of vessels in the vertebrata takes pkice in the 

 splanchnic mesoblast 5 but this appears to be due to the fact that the 

 circulation is at first mainly confined to the vitelline region, which is 

 covered by splanchnic mesoblast." 



Has it, however, been proved that a splanchnic layer covers the yelk 

 of fishes at a late stage, or after the inclusion of the yelk by the blasto- 

 derm? The reply to this is most positively in the negative in the case 

 of those forms devoid of a vitelline vascular system. In those types, 

 however, in which a vitelline system of capillaries is found, the answer 

 is not so clear. Sections of the salmon, just after hatching, are very in- 

 structive, and we here find an arrangement which is most interesting, 

 especially if those through the region of the liver be examined, from the 

 ventral border of which it is evident that vessels are continued directly 

 over the yelk, and that if they are not wholly channeled out of the thick 

 Plasmodium or yelk hypoblast they are at most covered on the external 

 side only, by an exceedingly thin layer of cells. Inasmuch as we know 

 that there are free nuclei imbedded in this Plasmodium or yelk hypo- 

 blast, is it not i)ossible that they may become the means of developing 

 cells for the walls of the vitelline capillaries as well as blood corpuscles? 

 As remarked some way back, I found that the external epiblastic soma- 

 topleural and outer peritoneal layers of the external yelk-sack of the 

 young salmon might be entirely stripped off from the yelk and that 

 they were nowhere adherent to it, and that this exposed the vascular 

 layer covering the yelk. Moreover, the space which lies between the 

 vascular and outer envelopes of the yelk has been derived from the 

 segmentation cavity and becomes abnormally and greatly distended 

 with water when salmon embryos are affected with what is known as 

 "dropsy" amongst fish-culturists. In such cases, too, the space will 

 often contain dead blood corpuscles, after some of the vitelline vessels 

 have been ruptured and injured, which often leads to the partial or 

 complete stoppage of the vitelline and hepatic circulations, which may 

 of course be fatal to the life of the embryo. The hind portion of the 

 outer sack is also sometimes abnormally distended backwards and is 

 finally constricted and sloughed off, while the embryo, which has lost a 

 part of the outer yelk covering in this curious manner, may go on de- 

 veloping normally if the place where the diseased part was broken off 

 has healed promptly. From all of these facts, it may be inferred that 

 whatever the significance of a splanchnoi^leural layer may be it cannot 

 in any case be other than the inner or lower peritoneal part which has 

 been reflected over the yelk and which is traversed by the vitelline blood- 

 vessels. Now in sections through just-hatched salmon, its tenuity is 

 very great and is present only as the thinnest kind of a film over the 

 true yelk hypoblast, but, as already stated, whether it may be certainly 

 identified with the innermost splanchnopleural layer is a question which 

 I cannot certainly answer. On the inner side of the vessels, the blood- 

 cells are seen to lie in immediate contact with the Plasmodium or yelk- 



