630 SKATES’ EGGS AND YOUNG. 
The inner part of the first closes up, while the outer remains open (fig. 
It is 
growth of the intervening parts, and still further by the extension of the 
pectoral fins forward between this remnant of the first fissure and those 
behind it, the former being thus thrown to the upper, and the latter to 
the under surface. The unclosed portion of the first branchial fissure is 
thus converted into the spiracle 
The transformation thus described is of very great interest when com- 
pared with the changes which occur in the corrésponding fissures of the 
m ind vertebrates, and enables us to establish an unexpected 
homology. Reichert, in his most important M of the devel- 
icis of the gill arches (‘visceral Bogen’) of the pig, has shown that 
in this animal the flrst fissure is gradually ipee from the others by 
the widening of the second arch, and for a time, even after all the others 
are closed up, forms a direct opening from the side of the neck into the 
hus be see at the spiracle is not only a true se e lastre in the 
Kos place, me pie in the end it is homologous with the Eustachian tube 
and the outer auditory passage wee these are separated from each 
other by the membrane of the tympan 
Professor Huxley, in a series of esami on the Vertebrate Skeleton, in 
which the homologies Ana development of it are discussed with great 
wu sets forth a t different view with regard to the formation 

chick, and he arrives at the same conclusions as Remak, leaving us to 
iufer that the auditory passage and Eustachian tube have no connection 
with the branchial fissures. We have gone over the same ground in the 
= anà have found Reichert’s observations, as mentioned above, fully 
rmed. 
Phe relation of the spiracle to the Daph fissures is still further 
shown by the fact that in some species, as in Po and Lemargus, it, 
like the others, is provided with respiratory fringes. jg the skate this 
is not the case, but in the adult a comb-like fold, fortia and prob- 
ably having the functions of, a gill, is found Lens within the spiracular 
. opening." 
For a ei iss of the development of the mouth and 
‘nostrils, and some other details which have been omitted in 
‘these extended quotations, the bondes: is referred to Profes- 
bd Wyman’s Memoir. 

