288° 
throughout the length of the body. The significance of this fact has 
yet to be determined. 
The hypothesis that the cardinal vein has much to do with the 
closure of the communication between the coelome and cavity of the 
somite was suggested to me by the appearance of the tissue as shown 
in Fig. 3. The ragged condition of the mesothelium on the inner side 
is very marked: the distinct edge has been lost and we see the cells 
escaping into the surrounding mesenchyma. The appearance is exactly 
what we should expect to find did we know that a gradually increasing 
pressure was being applied at the point the vein occupies: the meso- 
thelium seems to have been slowly forced inwards until the strain 
became great enough to rupture the inner wall. Ragu‘) has descri- 
bed in Elasmobranchs the formation of a smal] diverticulum extending 
inwards from the coelome. His figure (Taf. X, Fig. 4) shows that 
this diverticulum is situated in a position corresponding to the break 
in the mesothelium in the chick, and his description is certainly sug- 
gestive in this connection. ,,Der Boden und die Wande dieses Diver- 
tikels sind der Sitz einer sehr lebhaften Zellvermehrung: die neuge- 
bildeten Zellen schieben sich zunächst zwischen Chorda und Muskel- 
platte vor und drängen dadurch diese von der Seitenfläche der Chorda, 
der sie bisher angelegen hatte, ab. Diese Zellen bilden die erste 
Anlage der axialen Bindesubstanz oder des Sklerotoms‘ (S. 242). 
Now, whether we have here a repetition in the chick of the pheno- 
mena observed in Elasmobranchs; whether the escaping cells, as shown 
in fig. 3, are the „erste Anlage der axialen Bindesubstanz“ can only 
be decided by actual observation, but it certainly seems probable that 
those somites which have retained their primitive condition in relation 
to the coelome should also exhibit other primitive phenomena. 
Finally the question naturally occurs whether the communications 
which exist between these three somites and the coelome are the 
cavities which later appear as the three early openings of the Müllerian 
duct. Sep@gwick?) has shown that the cavities of the segmental 
tubules in Elasmobranchs are those which originally connected the 
somites and coelome, and, as the number of such connections and the 
number of original openings of the Müllerian duct appears to be the 
same, one is naturally inclined to connect the two facts. I have not 
followed out the point fully, but I do not believe that these connec- 
tions appear later as the Müllerian duct oppenings for the reason 
1) Morphologisches Jahrbuch, 15. Band. 
2) Quarr. Journ. Micros. Science, Vol. XX. 
