668 BASHFORD DEAN, 



as of undoubted phylogenetic value. For tlie i)rocess of shorteuing 

 phylogenetic stages in the ontogeny of a bony fish has evidently pro- 

 gressed to such a degree that the ancestral relations of epiblastic 

 Organs niight have become greatly obscured by the processes 

 of precocious growth. To regard the evidence of the common 

 sensory Anlage in Serranus as final and satisfactory evidence of the 

 genetic kinship of these structures would, in the opinion of the present 

 writer, be scarcely raore conclusive than the homologizing of hypo- 

 physis and teeth on the ground of their having arisen in Äniia as 

 more or less solid structures in the epiblastic roof of the mouth. 



Sucking disc. 



The mode of origin of the sucking disc gives the most interest- 

 ing evidence of how precociously embryonic and larval structures may 

 be developed. As far as histological evidence goes there is certainly 

 no diff'erence between the enlarged thick-walled cup-shaped organs 

 which arise on the snout of the late embryos of Ämia or of Lepido- 

 steus, and the typical pit organs, or sense buds, which later occur on 

 other integumental regions. It is found in fact that a gradation in 

 size exists which connects the huge sucking organs of the snout ^) 

 with the inconspicuous pit organs of the trunk. It is certain that 

 these sucking organs, whether in the condition of little differentiated 

 Haftscheibe (Acipenser), scattered sucking hillocks (Lepidosfeus), or in 

 groups as ring-shaped lobes {Ämia\ are purely larval structures, and 

 that they occur (probably) only in Ganoids. They have evidently no 

 place in the general study of the vertebrate head. But humble as their 

 morphological role may be they yet throw a strong side light on the 

 mode of evolution of structures not merely of the vertebrate head but 

 of the entire vertebrate organism. For if a process of evolution can 

 be made for so transient a purpose to produce sense buds in a 

 definite locality enormously enlarged, and at a very early period, 

 earlier not only than their kindred structures but even earlier than 

 mouth, nose, liver, fins, gill slits, a similar process of evolution can 

 even as evidently cause such sense organs as the nose, ear and eye 

 to appear precociously in a definite region and in such enlarged and 

 perfected form as to mask their genetic kinships. So too could brain 

 parts or cranial nerves be precociously evolved, so that even at their 



1) There is but little diiference histologically between these and 

 the neighboring nasal pits. 



