Paekee. — Anatomy and Embryology of Scyrnnus 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.l) 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 (br.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, Scymnus belonging to the Spinacidm, Scyllium and Pristiurus 

 to the Scyllid(B, 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.h, 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 (pc, 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 "0" of Balfour, although in correspondence with the fact that the 

 brain is less advanced, in comparison with other organs, than that of 

 Scyllium, the cerebral flexure and general features of the head correspond 



