SKATES' EGGS AND YOUNG. 62! 
fringes permanently attached to their branchial arches, which are not 
known to be replaced by, or to coexist with, internal gills. They are 
wcir sole organs of respiration, for their lungs are too imperfect and 
have much physiological importance. In frogs, toads, 
the external gills are replaced by internal ones, and 
e 
— 
rudimentary to 
and salamanders, 
these in turn by lungs. vie di 
Thus it will be seen that wis o 
Batrachian is perma- 
nently provided with in- 
ternal gills 
Sel Heus and Batrach- 
ians agree in this, that 
their embryos have in 
their first stages external 
fringes growing from the 
outer surface of the gill 
arches, and these fringes 
have the same structure 
in both. The Selachians 
still further agree with 
frogs, toads, and salaman- 
ders, in the fact that the 
outer Siegen are ab- 
by internal aeria odi 
ever, in the (oitwhig par- 
ticular. Selachians retain 
their internal gills perma- 
nently through life, while, 
if such exist at all in the 
Batrachians just men- 
tioned, it is only during 
the larval. stage, and ey Newly-hatched Skate; æ, yelk-sack in the cavity of the 
are soon replaced by abdomen, connecting w with the intestine, 6; c, embryonic 
: A t portion of the tail which disappears in the adult, and 
lungs.  Selachians may corresponds ith the portion of t 
^ : sal fins in the figures representing ‘still younger stages; 
therefore be said to pass a upper lip; e, developed facial disk. 
through stages analogous 
to the first and second stages of Anourous Batrachians and salamanders 
The other changes which the fissures pass through before the skate 
The seventh fissure is closed 

acquires its permanent form are as follows. 
up at a very early period, about the time that the dorsals are beginning 
to be formed. While first arch bends and is drawn forward as al- 
ready described in connection with the formation of the jaws, it at the 
Same time becomes broader, so as to widen the distance between the 
outh and the first fissure, or the second, after the first is partially closed. 
