[77] THE EVOLUTION OF THE FINS OF FISHES. 1057 



instead of directly from the primary layers during the earlier diplo- 

 blastic stage, as in the Pseudoccelia. 



The metameric arrangement of this secondary mesenchyme between 

 and in relation, above and below them, to the myotomes, is also, to a cer- 

 tain extent, a merely mechanical or physical phenomenon, because the 

 outgrowth of the gut-pouches in the primitive Bilateralia must have 

 given rise to the existing metamerism of Vermes, Artkropoda, and Ver- 

 tehrata. The bilateral symmetry of the myotomes and their antimerism 

 are, however, not always exact, as may be seen in both the young and 

 adult of Branchiostoma. This gives rise to disturbances in the symmet- 

 rical disposition of the secondary mesenchyme. 



The blood of all animals is evidently a mesenchymal tissue ; and it is 

 noteworthy that while the yelk of meroblastic vertt brate embryos is to 

 be aflfiliated with the hypoblast, in many cases the peculiar way in which 

 it is absorbed during development renders it mesenchymal. The cor- 

 puscles originating from the metamorphosis of the yelk substance are 

 actually transferred from a subhypoblastic position to a mesoblastic one. 

 This, as a number of observers have shown, is effected through an indi- 

 rect or direct communication between the heart and the surface of the 

 vitellus, from whence many if not all of the first blood corpuscles are 

 proliferated. 



XIV. — On the stimuli DETERMININa THE OUTGROWTH OF THE 

 LOWER LOBE OF THE CAUDAL FIN. 



The hypural lobe of the tail during its outgrowth has a tendency to 

 displace the chordal axis upwardly, as we have noted in the case of the 

 developing hypural cartilages of Alosa. Here the development of the 

 hypural elements is accompanied by a pressing inward of the ventral 

 wall of the chorda (Fig. 2, PI. 11). To what extent the upturning of 

 the chordal axis may be due to the deposition of material below it dur- 

 ing the development of the ventral lobe, which would tend to displace 

 that axis in an upward direction, it is not possible to state, but it is 

 fair to infer that the energy of growth in effecting displacement here 

 should be considered. But a hasty analysis of this question leads to 

 the conclusion that we are not merely to consider the effects of the out- 

 growth of the caudal hypural lobe, but the causes which led to such a 

 local hypertrophy of the median fin -fold as to originate this lobe, which 

 we have reason to think must represent a second hypaxial fin which 

 was derived by specialization or hypertrophy from part of a continuous 

 hypaxial series of rays, some of which were also atrophied. 



The development of this lobe is essentially similar to that of the other 

 median fins, the medullary portion of which we found was derived from 

 the mesoblast, some of which was pushed outwards into the primitive 

 median fin-fold of the lophocercal stage of development. As further 

 specialization was attained the local suppression of ray-bearing ele- 

 ments has taken place, so that those remaining have been exaggerated 

 H. Mis. 68 07 



