120 
ME. W. K. PAEIvEE ON THE STEECTUEE AND 
left side the umbilical vesicle is free, but on the right it is continuous with the blastoderm 
beneath the hyoid bar. At this point three things are clearly seen — the fore face of the 
auditory sac is chondrified, its outer part being pitted by the anterior canal, the hyoid 
bar has crept beneath this region, and the “primordial auditory involution” lies over 
the supero-external region of the sac; it is here cut through its middle. If this be 
compared with the other figures, especially figs. 2, 3, & 6, it will be seen that the naked 
upper part of the labyrinth lies just within the primordial opening in the skin. Hence 
I infer that the “ labyrinth ” itself has been constructed out of that layer which was pri- 
marily uppermost, the “epiblast,” and that the next layer within, the “mesoblast,” may 
be credited with the formation of the cartilaginous shield. 
The last of these sections (Plate II. fig. 2) belongs to the very close of the first stage, 
and is much in advance of the last ; it is also from a point further hack, although the 
broad hyoidean band has been displayed. Here the “ epenceplialic ” region is cut through, 
the third vesicle, and the medulla oblongata ; the apex of the notochord and of the 
“ basilar plate ” and the fore part of the “labyrinth” with its “shield” are severed; these 
parts have now coalesced beneath the rudimentary “ sacculus.” This band is a “ con- 
nective ” passing in front of the “ inferior fenestra” (fig. 4 ,fs.o.); it is very thin. On 
account of the natural obliquity of the parts, the first and second branchials are shown 
with their basal piece ; they are prematurely distinct in this specimen ; for I find them 
less specialized in others, which in other respects are more advanced*. The hinder 
part of the mouth here must still be “ epiblastic ” as to its lining, for it is in free com- 
munication, by the visceral clefts, with the outer skin. 
Second Stage . — TJnhatched Salmon with hyoid arch splitting up. 
Notwithstanding the fundamental identity of the morphological plan of growth in 
Vertebrate embros, the detail is marvellously varied, and these variations take place 
very early ; both the precocity and the amount of these variations, especially as seen in 
this and my former subject, the Frog, have surprised me again and again. The light 
obtained from that has indeed been useful for this ; yet not as a clear, flooding light, but 
merely as a lantern to my first steps. 
Therefore this piece of research has been made with a greater sense of comfort and 
confidence, but not with less caution and care. The manner in which the second post- 
oral arch is, as it were, chopped and split into fragments is very unlike what I have 
described in the Tadpole ; and more, it gives as yet little promise of becoming a “ Tele- 
ostean ” hyoid apparatus. The three foremost arches have not changed much, they are 
quite unsegmented ; but the second “ postoral ” (Plate II. fig. 3), besides having its fore- 
turned lower extremity cut off and rounded into a globular submesial joint, has been 
* The reader may think that I have drawn a little upon my imagination for the deep yelk-bed in which the 
auditory region is made to lie in this figure, and for the awkward manner in which the chorion is made to com- 
press the young cerebellum ; all I can say is that the actual object, uninjured in relation, although shaven 
through, appeared as strange to me. 
