April 1 6, 1 8 74 J 



NATURE 



467 



REPORT OF PROF. PARKER'S HUNTER/AA' 

 LECTURES ''ON THE STRUCTURE AND 

 DEVELOPMENT OF THE VERTEBRATE 

 SKULL " * 



II. 



IT is well known that the eggs of sharks and rays, 

 when deposited, are enclosed in a strong horny cap- 

 sule or "purse" formed as a secretion from the oviduct. 

 In both groups these curious appendages have the form 

 of a pillow-case, the corners being pointed in the rays, 

 and produced into long tendril-like processes in the shark 

 and dog-fish. The embryo remains enclosed in the purse 

 until about six months after oviposition, and it is during 

 this period that all the most important metamorphoses 

 are gone through. 



The youngest embryo described was nearly an inch 



P/ff.3 



Flc. 3.— Head of Embryo Dog-fish, 11 lines long. Lch. lachrymil cleft ; 

 C I, 2, 3, Cerebral vesicles ; Hm, Cerebral hemispheres ; inp, fronto- 

 nasal process ; Sp. Spiracle. Other references as before. The visceral 

 arches are dotted for distinction's sake. 



long, an extremely active little creature, attached to a 

 yolk-sac about ■] of an inch in diameter. In this stage 

 the head and branchial region are large and conspicuous, 

 the body slender, and tapering off to a long thread-like 

 tail. The dorsal lamina: have completely united, leaving, 

 however, a very thin covering to the hinder division of 

 the brain, which consists of the three primiry cerebral 

 vesicles (Fig. 3, C i —3), b;nt over the end of the noto- 

 chord in such a way that the second, or middle division, 

 forms the anterior termination of the head ; the " cerebral 

 flexure " is, therefore, complete. The future hemispheres 

 (Hm) have already appeared as small buds from the forc- 



rfff.4 



Fig. 4.— HcadofEinbryoDoT.fish, ijin. b.ng. References as before. 



brain (Ci). The nasal, visual, and auditory organs are in an 

 extremely rudimentary condition. On the under surface 

 of the head is the large square mouth, bounded above by 

 the fronto-nasal process (Inp), a shield-shaped elevation of 

 the integument between the nasal sacs, found in the em- 

 bryos of even the highest vertebrata, but persistent in 

 the sharks and rays. Beneath the eye, and communi- 

 cating by a slit running below the inferior boundary of the 

 fronto-nasal process, is a cleft (Lch) answering to the 

 lachrymal passage of the higher vertebrata, and formed 

 by the shutting off of a portion of the original first visceral 

 cleft by the growth of the pterygo-palatine arcade. This 

 cleft, persistent in the higher animals, is a transient struc- 



* Continued from p. 426. 



ture in the Elamobanchs. One of the most noticeable 

 features in the embryo at this stage is the presence of a 

 number of long filamentous external gills, each con- 

 taining a single capillary loop ; of these about ten spring 

 from the hyoid and each of the branchial arches, while four 

 much shorter ones project from the future spiracle, and 

 are attached to the mandibular arch. The internal 

 branchiae are at present functionless, and form mere cog- 

 like projections on the arches. 



The embryo at this age is so transparent, that the 

 visceral arches can be seen with sufficient distinctness 

 through the skin without any dissection whatever. Even 

 at such an early period, the anterior face-bars already 

 begin to show signs of segmentation, there being constric- 



JZ^rJl J£J3r.S , 



Fig. 5.— Skull of Ray. M Pt, Spiracular cartilage ; ihl, interhyal 



lig.-iment. 



tions in the mandibular and hyoid arches, where division 

 will afterwards take place. The upper part of these arches 

 has assumed the pedate form which is taken on at a 

 later period by the branchials. The pterygo-palatine 

 arcade is already as large proportionally as in the adult, 

 the true apex of the mandibular arch being reduced by 

 its outgrowth to a mere tubercle (M Pt). 



Granular subcutaneous thickenings have alre.tdy ap- 

 peared in relation with the face-bars ; these are the extra- 

 viscerals. In the same unchondrificd condition are the 

 parachordal and paraneural elements of the skull. 



Embryos of ij and li in. in length possess external gills 



-Head of Embryo Ray, 1 



Pn , Pineal gland. 



two or three times as long as in the preceding stage 

 those on the mandibular arch having already shrunk and 

 begun to form the pseudo-branchia ; the internal gills are 

 still functionless. The eye is completely formed. The 

 investing mass and the nose and ear capsules are chrondi- 

 fied, but the two halves of the former are still separate, 

 and the roof and walls of the cranium membranous. To 

 make out clearly the relations of the facial arches, it is now 

 necessary to dissect away the skin (see Fig. 4) ; it is then 

 seen that the process of segmentation has advanced 

 greatly, the arches behind ttie mouth being split up 

 as in the adult, and dift'ering from those of the full-grown 

 shark only in form. The trabecuki; have become llat- 



