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figure of a young Tergipes in which the gonads are shown 
to consist of paired follicles metamerically arranged and 
-alternating with the paired cerata (reproduced in BRONN’s 
Klassen und Ordnungen, Malacozoa, Plate 55). Very obvious 
is also the originally segmented structure of the gonads in 
DAVENPORT's (1893) figures. Alternating with the ento- 
dermal pouches, which here bear transverse rows of cerata, 
the genital follicles are situated, one pair of ovaria and 
one pair of testes between every two rows of cerata. 
1 myself was especially struck by the regular alternation of 
‘cerata and genital follicles, while studying a young Nudi- 
branch from the harbour of Helder, to which my attention 
had been drawn by its perfect transparency. It showed 
very clearly that the diverticula of the gut, applying them- 
‘selves to the ectoderm in the same way as gill-pouches, 
cause the formation of the cerata. 
It seems to me not improbable that the gill-pouches of 
Vertebrates may be directly compared to the gut diverticula 
of flatworms and Nemerteans and, perhaps, of Nudibranchs. 
In the axolotl HOUSSAY (1893, p. 33) described and figured 
very distinctly a double series of lateral diverticula of the 
gut reaching from the mouth to beyond the anus and alter- 
nating quite regularly with the myotomes. Only the fore- 
most break through and form the gill-slits. His observations 
for Siredon are confirmed by Miss PLATT (1894, p. 933, 961) 
for Necturus. These metameric outpocketings are very clearly 
defined and remind one vividly of the intestinal diver- 
ticula of the gut in flatworms, which alternate with the 
genital follicles. On the other hand, I do not think that 
from these observations it follows that gill-slits must have 
been present formerly along the whole length of the soma 
in Vertebrates, as assumed by BOVERI (1892). Combining then 
MEYER's above cited theory on the origin of metamerism 
(gonocoel theory), with the comparison just made, we have 
one more argument in favour of the view that branchiomerism 
Originally corresponds to mesomerism 
In studying ZIEGLER's figures, one gets the impression, 
that it is fairly obvious, that in Elasmobranchs also the 
Segmentation of the branchial region is simply the con- 
tinuation of that of the trunk. While in the adult the 
Series of gill-slits extends backwards considerably further 
than the corresponding region of the cranium, causing the 
glossopharyngeus and the branches of the vagus to take an 
