194 ME. W. K. PABKEE ON THE STEUCTUEE AJSTD DEVELOPJfENT 



embryos nearly an inch in length, the skeletal tissues were acquiring a greater density, 

 and the various folds of the skin were much more perfect. Yet over the third vesicle (C^) 

 the integument was quite transparent, and the contents of the head visible through it. A 

 lateral view (fig. 2), seen by reflected light in a specimen hardened both by alcohol and 

 chromic acid, shows how the skin is acquiring its proper characters in its growth from 

 below upwards. The sense-capsule folds are closing ; and the posterior edge of each 

 postoral visceral arch has developed an opercular " vallance," the fringe from the 

 second beuig the counterpart of that which is so largely extended backwards over the 

 hinder arches (branchials) of the Osseous Fish. Here the fiee labiate edge grows 

 from the mandible and gill-arches, as well as from the hyoid arch. Arrested at this 

 stage, they would leave the gills much exposed ; but they close in to a great degree, 

 leaving only the well-known branchial slits. Morphologically considered, they also are 

 the rudiments of such a growth of the skin as in an early stage covers over the visceral 

 clefts in the " Abranchiata." No cartilage is developed at present in the substance of 

 the mandibular arch at this part, merely a strong ligament which attaches the mandible 

 to the skull between the trigeminal nerve and the apex of the hyoid arch. This liga- 

 ment is the primary apex ; the pterygo-quadrate is the secondary fork. A deep fissure 

 is seen between the inturned end of the pterygo-quadrate bar and the olfactory sac {na). 

 During growth the arcuate cleft between the first and second postorals has become a 

 large triangular space, with the base above and the apex below. From the posterior 

 edge of the upper part of the mandibular bar four clubbed filaments proceed ; they 

 look upwards and outwards : these are the free external transitory gills of the mandi- 

 bular arch, the precursors of the pseudo-branchia. The counterparts of these, growing 

 out of the succeeding arches, all but the last, are four or five times as long as in the 

 more immature specimen. They are about ten on each bar, both on the right and left 

 side. The lower and upper views of this specimen (figs. 3 & 4) are very instructive ; 

 and if the actual form of the enclosed bars of cartilage be held in mind (see fig. 1), it 

 will be easy to understand their structure. The first cerebral vesicle (C^) is completely 

 beneath the second {C~); and beneath the first the curious nasal sacs are seen, with their 

 sigmoid valvular opening. The trabecular plate shows its form even in the opaque 

 object. In front of the mouth are seen three lobes : the paired lobes contain the soft 

 bulbous ends of the trabecular bars ; and the azygous elevation between the nasal sacs 

 contains the prsenasal or basitrabecular cartilage, an unpaired commissural bar uniting 

 tlie distal ends of the trabecular cornua. The solid side walls of the mouth contain not 

 only the pedate pterygo-quadrate bar, but also the fourth upper labial cartilage ; this 

 will be shown better in a more advanced stage. 



The lower jaw, seen from beneath, is a quadrilobate mass fixed behind and below by 

 a broad short pedicle. Its external lobes are the angular and articular regions ; and the 

 submesial swellings contain the short Meckelian regions of this peculiar mandibular 

 arch. The following arches are much more bowed out ; and between them the visceral 



