The Development of the Cartilaginous Skull ete. in Necturus. 457 
Summary of chief results. 
Part I. a) The branchial cartilages and the anterior part of 
the trabeculae arise in tissue of ectodermic origin. 
b) The basal plate of the skull, the auditory capsules, and the 
occipital arch are of mesodermic origin. 
The conclusions reached in Part I are discussed in the Intro- 
duction. 
Part Il. a) The plate of prochondral tissue which primarily 
underlies the brain is not coextensive with the basal plate of the 
skull, but includes in its antero-median part prochondral tissue 
surrounding the point of the chorda, which is not converted into 
cartilage. 
b) Distinctly separate cartilaginous elements, the homologues of 
the independent trabecular and occipital plates found in Triton, do 
not appear in Necturus in the development of the basal plate, 
although a middle region of the prochondral plate chondrifies slightly 
later than the anterior and posterior regions of the plate. 
c) The operculum auris arises near the antero-ventral margin 
of the fenestra ovalis in cells of the mesenchyma external to the 
auditory capsule. 
d) The dorsal part of the crista trabeculae arises independently, 
and is possibly the rudiment of the large alisphenoid cartilage found 
in the Selachii. 
e) A rudimentary arch, shown by its relation to the myotomes 
to be serially homologous with the occipital arch and the neural 
arches of the trunk, is taken into the otic capsule. 
f) The cartilage roofing the posterior part of the brain, tectum 
interoccipitale, arises independently from paired Anlagen and 
becomes connected with the dorsal extremities of the occipital and 
praeoccipital arches, as the cartilage roofing the spinal cord, which 
also arises from paired Anlagen, is connected with the neural arches. 
Part III. a) Each of the proper branchial muscles may be 
traced to mesothelial tissue primarily continuous with the wall of 
the pericardium, and is consequently formed morphologically in the 
wall of the coelom. No branchial muscle in Necturus arises in 
tissue similar to that composing the »periaxiale Stränge« described 
by Goronowirtscu as taking part in the formation of the branchial 
musculature in the bird. 
b) The first postotic somite gives rise to no muscle fibres, 
