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1876. ] The Vertebrates and Invertebrates. 599 
the body-cavity is finally closed up on the side of the body oppo- 
site to the nervous system, point to a common origin at a com- 
paratively high level. The surface of the animal which is called 
ventral is determined by the presence of the mouth on that sur- 
ace; and if any vertebrates had a mouth-opening between the 
brain and the spinal cord on the dorsal surface, that dorsal sur- 
face would necessarily become ventral. Since, moreover, the 
ancestors of the vertebrata must have had a nervous ring sur- 
rounding their gullet, it would appear more reasonable to sup- 
pose that the mouth-opening had been changed in the course of 
development than that the situation of the nervous centres had 
been altered. We are thus led to look for traces of an old 
mouth-opening on that surface of the early vertebrates which 
corresponded to our dorsal surface, and to seek reasons for re- 
garding our present mouth as a comparatively modern develop- 
ment. 
Dr. Dohrn believes that the old mouth passed through the 
nervous centres between the crura cerebelli, or, more accurately, 
in the fossa rhomboidea, or fourth ventricle, which is remarkable 
for being of greater proportionate size early in development, and 
afterwards undergoing retrogression. At an early stage we only 
need to conceive a slit to be made in the nerve tube at the bot- 
tom of the fossa rhomboidea, in: order to furnish a suitable pas- 
sage into the alimentary canal. His first reason for regarding 
the vertebrate mouth as a modern structure is that it arises so 
extraordinarily late in development. The embryonic body is 
almost completely framed, all the great systems are established, 
the circulation is in active operation, while as yet there is no 
mouth. Again, the mouth does not arise in the position in 
which it permanently remains in the great majority. It under- 
goes considerable shifting forwards. Only in the Selachians and 
Ganoids does it retain its primitive situation. Moreover, the 
study of development is steadily tending to establish the idea 
that the mouth of vertebrates is homodynamous with the gill- 
clefts. It is limited, like them, by a pair of arches, lies just in 
front of the first pair of gill-clefts, arises simultaneously with 
them in the embryo, and opens into the alimentary canal. A 
glance at the ventral surface of a Ray shows the likeness of the 
mouth to a pair of coalesced gill-clefts. Consequently, it becomes 
Probable that the present mouth-opening once existed and func- 
tioned as a gill-cleft ; that at a certain period in the ascending 
development, both the old and the new mouths supplied nourish- 
