1890.] Embryology. l 491 
The actinotrichia are at first joined together by a thin cuticular 
membrane; later, they separate more widely, and tne intervening 
membrane between the actinotrichial fibres disappears. The latter 
also are not round at first, but flattened, or oval in cross-section. Their 
greatest diameter also corresponds to the direction of the plane of the 
membrane of which they form a part. 
To the proximal ends of the two rows of actinotrichia the fin muscles 
Chimeroids. This insertion of the fin muscles, their serial action from 
before backward, as well as the necessary undulatory motion of the tail 
from before backward in the act of locomotion, must throw the fin folds 
into a lateral undulatory motion from before backward. This undula- 
tory motion would tend to favor the breaking of the cuticular membrane 
under the epidermis of both sides of the fin into parallel threads, owing 
to the short flexures into which its substance must be thrown, In this 
way the genesis of the actinotrichia themselves may be traced to the 
direct action of physical causes. The subsequent cross-fracturing of 
the rays derived from a further development of the actinotrichia, I have 
elsewhere proved to be due to the interaction of the organism and the 
resistance offered by the surroundings to the motions of the fins. ! 
The thickness of the continuous subepidermal basement membrane in 
Batrachus, also varies in a singular and suggestive way. It is decidedly 
thicker on the dorsal aspect and on the upper portions of the sides of the 
body, and thinnest on the ventral aspect whichis most protected in this 
form, which, as is well known, lives by preference at the sea-bottom 
resting on the mud. 
In the earlier forms of fishes, such as Coccosteus, Mycterops, Pteras- 
pis, Cephalaspis, etc., there was a tendency to form an unbroken dorsal 
cephalic shield. This tendency is still preserved in the evolution of the 
cranial plates of the sturgeon on the sides and top of the head, 
. and is expressed on the body, in that the dorsal scutes are always the 
first to be developed. The development of the cranial exoskeleton in 
primitive forms, and in now existing types representing the latter, 
therefore coincides with this early appearance of traces of the superficial 
skeletal matrix in a subepidermal position on the dorsal aspect of the 
head and body in the existing larve of unarmored forms, which 
may be supposed to have lost such a defensive exoskeleton. The par- 
allelism here pointed out is, at any rate, extremely suggestive, and if 
capable of further demonstration will show how persistently an extremely 
ancient character tends to be inherited. - Joun A. RYDER. 
1 Proofs of the effects of Habitual use in the Modification of Aaaa Organisms. Proc. 
Amer. Philos. Soc. Vol., XXVI., 1889. 
