Parxer.—Anatomy and Embryology of Seymnus lichia. 231 
In other characters, however, this embryo had advanced considerably 
beyond “I,” and was indeed as far advanced as ** M." The mouth (m) is 
large, and its thickened edge due to the presence of the pterygo-quadrate ( pt.q) 
. and Mechelian (mn) bars which afterwards become the upper and lower jaws, 
is very obvious. There is already the full number of six visceral clefts, of 
which the first (sp) has completely taken on the character of a spiracle: on 
the anterior edge of the second (first branchial) cleft (br.1) are minute den- 
ticulations, which appear to be the rudiments of the external gills. Lastly 
the pectoral fins (pc) are well developed, occurring in the form of small out- 
growths a short distance behind the last gill-cleft (7.5). 
It is thus seen that the mouth, the gill-clefts, and the pectoral fins 
develope far more rapidly in Scymnus than in either of the genera (Scyllium 
and Pristiurus) studied by Balfour. A Scymnus embryo of stage ** I" has its 
pectoral fins as far advanced as a Scyllium or Pristiurus of stage ** L,” while 
its gill-clefts are in the condition of those of the same genera in stage “M.” 
Further observation will be necessary to show whether this is a constant 
family difference, Seymnus belonging to the Spinacide, Scyllium and Pristiurus 
to the Scyllide, or whether the embryo I have just described is abnormal. 
I have noticed more than once in Mustelus antarcticus one foetus out of the 
whole number in a single gravid female in a far more backward stage of 
development than the rest, and such arrest of development is not unlikely 
to be accompanied by deformities of some sort. 
Second stage.—To this, as to the first stage, only one of the embryos 
belonged ; it was about 18 mm. long, and is shown in fig. 12. 
It is, on the whole, intermediate between Balfour’s stages “M” and 
“ N,” inclining in most respects to ** M." The head has a remarkably square 
outline in side view, the cerebral flexure having proceeded just far enough to 
bring the fore- and mid-brains (f.b, m.b) into the same transverse vertical 
plane. The eye (e) is large, and the nostril (na) well formed. The distal end 
of Mechel’s cartilage has rotated forwards to such an extent that the axis of the 
mandible (mn) is nearly vertical. The rudiments of external gills are visible 
in all the branchial clefts but the last: none have as yet appeared in the 
spiracle (sp). 
All the fins (pe, pv, d 1, d 2) are now formed, and occur in the form of 
flattened crests, mostly with evenly curved free edges: the caudal fin (c) is 
perfectly diphycercal. The somatic stalk has undergone great relative re- 
duction. 
Third stage.—The remaining eight embryos correspond pretty nearly with 
stage “O” of Balfour, although in correspondence with the fact that the 
brain is less advanced, in comparison with other organs, than that of 
Scylliun, the cerebral flexure and general features of the head correspond 
