[85] THE EVOLUTION OF THE FINS OF FISHES. 1065 



by the absorption of water, or the formation of oil, or by the incorpora- 

 tion of large amounts of plasmic matter is remarkable; yet there are 

 phenomena met with which are apparently the very reverse of anj'tbing 

 comparable to phagocytic;; these are such as show minute granular 

 bodies developed outside of the vitellus and between it and the egg 

 membrane, as occurs in the case of Amiurus albidus,* in which the gran- 

 ules are quite free. In the young Spanish mackerel somewhat similar 

 granular bodies lie beneath the skin and represent a homogeneous 

 stratum found largely developed in the fin-folds of the embryos of 

 certain forms, as in Alosa, for example. C.Emery first called attention 

 to the existence of this homogeneous substance in the fin-folds of the 

 lophocercal stages of young Teleosts in a memoirt published about a 

 year since. He regards this substance as a secretion; that is, it is 

 mesenchymal, and may be derived from the mesoblast, the epiblast also 

 taking a share in its formation, and may possibly be comparable to the 

 body of the gelatinous substance found in the umbrella of Medusae. 



This homogenous stratum precedes the cellular mesoblast in its advent 

 into the fin-folds of fishes, but it may be doubted if it has any influence 

 in determining heterocerey further than that it may supply the ma- 

 terial which is eventually converted into cellular mesoblast. In thus- 

 tracing the causes and phenomena which are directly concerned in in- 

 ducing heterocerey, or which collaterally throw some light upon it, it 

 becomes very evident that it is a very complex problem, but this is no 

 reason why its elucidation should not be attempted. 



XV. — On the MOVEMENTS OF PARTS OF LIVING BODIES CONSIDEREB 

 AS THE CAUSES OF MORPHOLOGICAL DIFFERENTIATION. 



It is noteworthy that the LjiHfera is the only group of Vertebrates in 

 which the termination of the axis of the body is commonly and fixedly 

 upturned, and this, together with the usual development of mesial 

 basalia for the support of the true fin-rays, which basalia extend out- 

 ward usually to the level of the integument covering the body of the 

 adult, and in contact or not in contact proximally with apophyses of the 

 vertebrae, constitute two of their most frequent, though not absolutely 

 constant, characters. The dorso-ventral symmetry of the terminal 

 caudal skeleton is thus more or less impaired because of this departure 

 from the usually almost rectilinear form of the vertebral axis of the 

 Ichthyopsida and the Lacertilian phylum of the Saur&psida. 



The other groups of Vertebrata present difi"erentiations of their ver- 

 tebral axis, which may be regarded merely as other departures from the 

 nearly rectilinear series of vertebral segments of the lowest Vertebrates. 



* Preliminary notice of the development and breeding habits of the Potomac Cat- 

 fish, Bull. U. S. Fish Comm., Ill, 1883, pp. 225-230. Such grannies are also found 

 floating inside the egg-membrane and around the embryo of Thymallus. 



t Sulla esistenza del cosidetto tessuto di secrezione nei vertebrati. Atti dellaR. Ac. 

 deUe Sci, di Torino, XVIII, 1883. 



