1890.] Embryology. 489 
EMBRYOLOGY. 
The Continuity of the Primary Matrix of the Scales and 
the Actinotrichia of Teleosts.—In Batrachus tau, if transverse 
sections are prepared from embryos a few days after hatching, it will 
be found that a basement membrane underlies the whole of the epi- 
dermis, as an extremely thin layer, This layer of ‘homogeneous matter 
covers the entire larval fish, and is interposed between the organs of 
mesoblastic origin and epiblast everywhere. In the fin folds there are 
linear and parallel thickenings of this thin lamina of basement mem- 
brane which eventually become the actinotrichia, as I have named the 
primary fibrous rays of all fishes. The mesoblastic cells which aid in 
developing the actinotrichia have a Uisposition to lengthen in a direc- 
tion parallel to the latter. 
The membrane over the rest of the body is continuous in Batrachus, 
and doubtless represents the matrix of scales in other forms. Its con- 
tinuity shows that the matrix of scales was probably originally contin- 
uous in all fishes, and that the lateral bendings of the body in locomo- 
tion have been in part the efficient cause of the segmentation of the 
continuous layer, into, at first, oblique bands as in Callichthys, and 
then into lozenge-shaped tesseræ. These bands doubtless conformed 
in direction at first, in some cases, to the underlying muscle plates 
Later secondary modifications introduced other complications. 
This evidence is at any rate highly interesting as pointing to the 
conclusion that the primary basis of the exoskeleton of fishes is con- 
clusious, so as to form an unbroken cuticular investment of the entire 
mesoblast. 
It also affords striking confirmation of my hypothesis! that the 
exoskeleton is to be traced phylogenetically to an uninterrupted cutic- 
ular or basement membrane occupying the position of the mesogloea 
in ccelenterates such as the Medusae and Hydroids. 
Aggregations of this cuticular membrane constitute the basis of the 
“cementum plates’ of the teeth of fishes, The ganoin and variously 
modified enamel of ganoid scales is a derivative of the epidermis proper. 
The cementum plates grade very gradually into the various types o 
osteodentine, and into that form of dentine with ramifying tubules seen 
in some ganoids and known as cosmin. Such aggregations or local 
thickenings of the cementum have arisen in the first place as the con 
1 A physiological theory of the calcification of the skeleton. Proc. Am. Philos. Soc., 
Vol. XXVI, 1880. 
