42 On the Embryology of the River- Lamprey . 



extends backwards in similar dimensions. The entirely un- 

 differentiated cells surrounding- the cavity assume an epithelial 

 character. In the canal which becomes the branchial cavity 

 lateral evaginatious appear, which extend from in front back- 

 wards. The evaginations become so pronounced that the 

 endoderm cells come into contact with tiie epitlielium of the 

 integument, which is finally absorbed. Opposite the evagina- 

 tion a slight depression is sometimes noticeable. At certain 

 spots it seemed to me that the epithelium of the future slit has 

 thrust itself so far outwards that the cells of the epidermis here 

 appear thinner. The groove which arises on the under surface 

 of the branchial cavity, in the neighbourhood of the first to the 

 fifth slit, and which subsequently becomes for the most part 

 closed off — the thyroid gland — has been described very fre- 

 quently, and 1 have nothing material to add. The main 

 features of the formation of the mouth, the union of the ecto- 

 dermal pouch with the endoderm of the branchial cavity, are 

 likewise well understood. If we examine a few longitudinal 

 sectionSj which were made parallel to the ventral surface of 

 the embryo, pretty much such as those which are shown by 

 A. Dohrn in his plate iv. (Bd. vii,), we are struck by the 

 similarity between the gill-slits and the two outer angles of 

 the oral pouch. It seems as though the latter are only the 

 end of the series of those slits. The similarity appears not 

 only in the position, but also in the form. 



The liver is formed by evaginations of the intestinal wall. 

 We observe that this is in connexion with canals^ which soon 

 become converted into tubes. At the spot at which these pro- 

 cesses are taking place the intestine is less abundantly sur- 

 rounded by yolk-cells than in its posterior division. At all 

 events the yolk-cells take no part in the formation of the 

 liver. In spite of Kupff'er's statement that a neurenteric 

 canal is not formed in the Lampreys, 1 must most empha- 

 tically insist upon its occurrence in these animals, though it 

 is true that at present not a single satisfactory drawing of it 

 exists. 



If wx follow the spinal cord in longitudinal sections, we see 

 with the greatest distinctness how the end of it bends round 

 the notochord, and then runs for a space beneath it, to end as 

 a somewhat thinner cord in the upper wall of the end-gut 

 above the anal opening. My preparations were stained with 

 ha3matoxylin, and the cells of the spinal cord (as also the 

 neurenteric canal) appeared very dark, in consequence of 

 which they stood out from the rest. The cells were, more- 

 over, much larger than others lying in the neighbourhood, 

 such as, for instance, those of the epithelium. The shape of 



