626 SKATES’ EGGS AND YOUNG. 
the fins are soon nearer the end than the middle: At the time of hatch- 
ing, the terminal portion is still present (fig. 96, c) ; but subsequently it 
is either absorbed, or, what is not improbable, is covered by the exten- 
sion of the dorsals backward. 
The anal fins, the first of which, as vietato stated, attains to a remark- 
ably large size, are gradually abso ; and are wholly removed before 
Fig. 91. iii e of gestation. From the fact of 
these fins having a temporary existence 
in the skate, and a permanent one in 
many sharks, it is not improbable that 
they may be ie in the embryos of 
all Plagioston 
This sel gaia! temporary exist- 
ence, and early removal of the anal fins, 
gives us another interesting example o 
the formation of parts which have no 
obvious use in the economy, and which 
must be regarded as having merely a 
morphological value. It falls into the 
same category with the caudal fin of the 
Embryo of Skate at the Shark- €mbryo of Pipa,* which is never used, 
CUM rsa fusi d d, the teeth of certain Cetaceans, the in- 
ferior incisors of the female Mastodon, 
which are all removed without being used, and the milk incisors of tke 
Guinea-pig, which are shed in utero. 
There is still another point of interest in the morphology of the tail of 
the species we are here considering; for although symmetrical, it does 
not at any period assume the heterocereal 
form, but retains permanently its primary 
embryonic or protocereal condition. 
this respect the skates hold a lower 
position than the sharks, nearly all 
whom pass through the protocereal into 
the heterocereal stage 
The pectoral and ventral fins begin as 
slight ridges on either side, but each soon 
takes on the form of a half" oval disk (fig. é 
88, a, b). At first the two are nearly ‘Head of fig. 91, enlarged. a, nasal 
continuous in the same plane (fig. 88), lobe; 6, facial disk; c, upper lip. 
but the pectorals (figs. 94 and 95, a) 
grow the most rapidly, gradually assume a somewhat oblique position, 
and in a short time partially cover the ventrals. tine of the specimens 
were of a proper age to show whether or not the pectorals were formed 
first, as is the case with the fore limbs of. all the vertebrates whose de- 
xu c o M d iu i M 


^ 

*The singular Surinam To: 
P. 
PAM rw ad whose eggs are developed in cells formed on the back of the 
