38 M. rii. Owsjannikow on the 



The whole of the earliest sense- organs appear to have arisen 

 in the same manner tliroughout the entire Vertebrate phylum. 



The Heart. 



I have observed the earliest rudiments of the heart, or, 

 rather, of the venous vessel in embryos of the age of 133 

 hours. The longitudinal section taken from above down- 

 wards exhibits an arrangement somewhat similar to that 

 figured by A. Goette in his well-known work on the develop- 

 ment of the fire-bellied toad (fig. 37) ; for we observe that 

 the enteric cavity has attained its greatest dimensions in the 

 region of the head, which is in process of formation. Poste- 

 riorly towards the dorsal surface it becomes narrower and 

 forms a very slight indentation in the middle of the undiffe- 

 rentiated hypoblast cells, and then proceeds as a sac-shaped 

 depression in the yolk in the direction of the ventral surface. 

 The walls both of the enteric cavity and of the venous sinus 

 are clothed with cells of the endoblast, which as yet are far 

 from having assumed an epithelial character. They are large 

 and full of yolk-granules. At this period we have only the 

 cavity of the vessel before us, which subsequently becomes 

 constricted off from the intestine. The appearance of the 

 rudiment of the venous system alters very little during the 

 next forty or fifty hours. 



In embryos of 180 hours the body has attained a consider- 

 able length. In longitudinal sections we observe the 

 branchial cavity in the form of a, long canal which already 

 possesses gill-slits. Below the branchial cavity there appears, 

 as in the previous stage, a depression passing off in a lateral 

 direction from the intestine. It has in the meantime become 

 somewhat longer and its lower end less regular. As a matter 

 of fact it is continued in the shape of a cleft, which can be 

 traced a very long way backwards, while rifts may be 

 observed proceeding from it in all directions, which finally 

 lose themselves between the yolk-spheres. 



It is evident from what is here seen that long before the 

 development of the heart we have a system of canals which 

 are filled with a fluid — the lymph. In all the cavities men- 

 tioned, in the cavity of the intestine, the branchial chamber, 

 &c. we find circular rings, a kind of membranes, which are 

 ]jrobably remnants of yolk-spheres which have undergone 

 dissolution. Isolated channels of this kind are to be met with 

 in which some few yolk-graimles are still present, while others 

 are quite full of them. The granules are much smaller than 

 the neighbouring cells or yolk-spheres of the undifferentiated 



