bad 
1891.] Recent Studies of the Vertebrate Head. 97 
mentary somites, to the caudal of which apparently belongs a 
nerve of transitory duration. Froriep had already discovered 
four somites in the occipital region of the chick, but it will be 
seen that in the lizard there are five somites in this region. The 
two cephalic roots of the hypoglossus possess neither ganglia nor 
dorsal roots; the condition of the third root in this respect has 
not been determined; the first cervical nerve has a transient gan- 
glion, and the second cervical a permanent ganglion. The hypo- 
glossus thus seems to represent a complex of true spinal nerves, 
whose ganglia and dorsal roots have partially or completely 
degenerated. According to Hoffmann (’89), on the hinder portion 
of the lizard’s brain appears an evident segmentation. Other 
authors had previously noticed this. Hoffmann finds in the hind- 
brain and medulla seven segments. From the caudal of these 
springs the X. nerve; from the next, or sixth, the IX.; opposite 
the fifth is the ear vescicle; from the fourth arises the VIL-VII. : 
from the third none; from the second the V.; from the cephalic 
border of the first segment the trochlear nerve primarily takes its 
origin, though later shifting over to the midbrain. 
Rabl (’89) considers the vertebrate head as consisting of two 
regions: a cephalic or Bo unsegmented, and a caudal or 
distal segmented region.! The ear vescicle forms the boundary 
between the two portions, but is to be reckoned with the prox- 
imal. The mesoderm of the proximal section may be divided 
into segments which neither in mode of origin nor in further de- 
velopment can be compared with protovertebre. The five distal 
somites arise exactly as the protovertebre. The first protoverte- 
bra to appear is the fifth head somite of Van Wijhe, or the first 
distal somite. The musculature and connective tissue of the dis- 
tal somites develop in the same portions as in the protovertebre 
of the body. Dorsal and ventral nerve roots occur in this region 
as in the body. In their origin the proximal somites show scarcely 
even a distant relationship with the structure of protovertebre. 
The proximal somites cannot be called primary, for they appear 
later than the Tans The muscles of the proximal re- 
l Kastschenko had d obse lin b r p sad did the meso- 
derm of the anterior portion of the head appear segmented 
