58 JOURNAL OF. THE 
nized that this series of organs differs at any rate in the 
matter of distribution, from the segmentally arranged 
organs extending alone the side of the body of certain 
annelids (Capitellidae, Kisig’; leeches, Whitman’. ) 
The small number and local distribution of the seg- 
mental sense organs of the vertebrate embryo, as com- 
pared with the condition after the lateral line has become 
well developed, suggests inevitably the question: If so 
much of the metameric character of these organs in the 
adult (or larva) is clearly secondary, how decply seated 
is, the metameric arrangement of the primitive (few, em- 
bryonic) branchial sense organs? Is it possible that this 
series of organs has arisen from a single pair (one on each 
side of the body), as some (Brooks’) believe the gill clefts 
to be traceable to a single pair? 
There exist certain observations which it is possible to 
interpret as supporting this idea. In 1891 I described* 
for a teleost (Serranus embryos) a sensory anlage on the 
side of the head. The reference to these observations in 
Minot’s Human Embryology (1892) is succinct, and indi- 
cates the state of our knowledge on this point at the time: 
“This thickening (the anlage on each side of the head) 
torms a long shallow furrow, which subsequently divides 
into three parts, of which the first becomes a sense organ 
over the gill cleft, the second. the auditory invagination, 
and the third, the anlage of the sense-organs of the lat- 
eral line. This peculiar development confirms the notion 
that all these organs belong in one series, but the appear- 
ance of a continuous thickening as the anlage of them all, 
has as yet been observed only in this fsh, and may not 
indicate a corresponding ancestral condition. Unfortu- 
§ Die Segmental-organe der Capitelliden. Mitt. a. d. Zool. Sta. zu 
Neapel, 1879. 
6 Some New Facts about the Hirudinea. Journ. Morphology, 1889, 
7The Genus Salpa. Baltimore, 1893. 
8 Embryology of the Sea Bass. Bull. U. S. Fish Comm. Vol: IX, 
