OBSERVATIONS ON THE DEVELOPMENT OF ItAIA B ATIS. 3$ 



effected in part by the more rapid gro* h of that portion of the tail whiflh is in 

 front of them, while that which is behind scarcely increases in size, and thus the 

 fins are soon nearer the end than the middle. At the time of hatching, the terminal 

 portion is still present (fig. 11, c); but subs, .piently it is either absorb 1, or, what is 

 not improbable, is covered by the extension of the dorsals backward. 



The anal fins, the first of which, as already stated, attains to a remarkably lar^o 

 size, are gradually absorbed, and are wholly removed before the end of gestation. 

 From the fact of these fins having a temporary existence in the skate, and 

 manent one in many sharks, it is not improbable that they may he present in the 

 embryos of all Plagiostomes. In the torpedo the dorsal fins seem to retain their 

 primary or embryonic position, as they remain permanently in the middle of the 

 tail; and Uraptera has the slender terminal portion behind the dorsals persistent, 

 just as in the newly hatched Rata batis. 



This development, temporary existence, and early removal of the anal fins, 

 us another interesting example of the formation of parts which have no obN io 

 in the economy, and which must be regarded as having merely a morphological 

 It falls into 



a per 



g IV 



the same category with the caudal fin of the embryo of Pipa, which 



mas 



never used, the teeth of certain Cetaceans, the inferior incisors of the female 

 todon, which are all removed without being used, and the milk incisors of the Guinea- 

 pig, which are shed in utero. 



There is still another point of interest in the morphology of the tail of the species 



here 



considering; for although symmetrical, it does not at any period 



the heterocereal form, but retains permanently its primary embry 



pi 



condition 



In this respect the skates hold a lower position than the sharks, nearly all of 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. 4, a, b). At first the two are nearly 



continuous in the same plane (fig. 4), but the pectorals (figs. 8 and 9, a) grow the most 

 rapidly, gradually assume a somewhat oblique position, and in a short time partially 

 cover the ventrals. None of the specimens were of a proper age to show whether or 



the pec 



formed first, as is the case with the fore limbs of all vertebrates 



whose development has been thus far studied. As they grow, they advance on either 

 side of the head in the form of horns (figs. 8 and 9, a'), but by degrees the space 

 between these horns and the side of the head is filled up, and thus the eyes and the 

 persistent portion of the first branchial fissure are pushed to the upper surface, and 



