in the Fcetus of Vertebrated Animals. 93 



blood, is almost immediately put a stop to by the removal of 

 oxygen from the medium in which they are placed, before 

 any particular part of the embryo is formed upon which the 

 changes of respiration are afterwards more immediately produced. . 

 In the early stages of development, then, there appears to be . 

 what may be called a General or Interstitial Respiration *, or a 

 change essential to life, produced by oxygen in all the siib- ; 

 stance of the embryo, or of its accessory parts, which, as the 

 foetus is more perfectly formed, takes place in particular organs 

 only. As soon as a peculiar nutritive fluid, and a central pro- 

 pelling organ, are produced, this fluid is exposed on the ex- 

 panded surface of the yolk, to the influence of the respira- 

 tory medium, either directly, or through the coverings of the 

 ovum. \ 



Development of the Respirator?/ Organs in the Foetus of ^ 



Fishes. 



The sac of the yolk, the principal respiratory organ of the 

 foetus of fishes, difi'ers considerably in its relations in the Osseous 

 and Cartilaginous tribes. 



In the foetus of osseous fishes, as in that of the Blennius vi- 

 viparus, described by Rathkef and Forchhammer J, the yolk, 

 after it has received a covering from the expanded layers of the 

 germinal membrane, hangs like a loose bag from the abdomen, 

 and is connected by a narrow opening with the anterior part of 

 the intestine. The vascular network spread over, this covering 

 of the yolk at a later period, as has previously been remarked,' 

 is formed entirely of veins in osseous fishes. .. A branch of the 

 mesenteric veins (PI. III. Fig. 1, y) running along the back, part 

 of the abdomen, dips down to join the yolk at the place where this 

 sac is connected with the intestine ; this vein is divided into nu- 

 merous minute ramifications on the back part of the yolk, and 

 its capillary vessels unite below with those of another vein oc- 

 cupying the anterior side («/«/), and conveying the blood 



• See Geoff. St Hilaire on this subject, in the Memoires du Museum, 

 torn. X. 



t Geschichte des Embryo der Fische, in Burdach's Physiol. B. 2. S. 201. 



X De Blennii Vivipari formatlone et evolutione. Kiliae, 1819. 



