4e 
exactly the same conclusion regarding the length of the 
head of Heptanchus on embryological grounds. After having 
treated of the development of the skull of Acanthias he 
writes concerning that of Heptanchus: “Es sind aber deutliche 
Zeichen vorhanden, welche darauf hinweisen, dass die 
Elemente zweier Wirbel mit dem Schädel verschmolzen 
sind, so dass die beiden letzten Occipitalnerven dieses Tieres, 
welche FüRBRINGER mit y und z bezeichnet, eigentlich 
Spinalnerven (Occipitospinalnerven) sind. Erst der Nerv x 
muss mit dem oben mit z bezeichneten Nerven, der bei 
Acanthias den primitiven Occipitalbogen durchbohrt, homo- 
logisiert werden.”!) A better confirmation of our conclusions 
concerning the nature of the hypoglossus and its relation 
to-the gill-slits could not be given. 
Primarily and secondarily epibranchial somites and ner- 
ves. — FüRBRINGER, though at another place (l. c.p. 682, 
685-686) recognizing the general homology of cranial and 
spinal ganglia and rejecting emphatically the idea of an 
incongruency between branchiomerism and mesomerism, yet 
accepts FRORIEP's distinction of a cerebral and a spinal 
region in the head and the idea that all the myotomes 
belong to the latter, that is to the trunk. Thus he considers 
the epibranchial muscles, which are to be derived from 
the occipital myotomes, as muscles of “spinal”, that is 
primarily post-branchial, origin just as the hypobranchial 
muscles but in opposition to the primordial branchial muscles, 
the constrictores which he calls “cerebral” muscles and which 
accordingly are considered as belonging to FRORIEP's 
primarily unsegmented head-region. In the same way the 
occipital nerves must then be considered as spinal nerves. 
n this we can not altogether agree with him. Just aS 
in Petromyzon where it has been proved by the study of 
ontogeny, part of the epibranchial musculature of the adult 
is no doubt of post-branchial origin and has become 
epibranchial only by the secondary extension of the branchial 
basket. VAN WYHE (1889, p. 558) demonstrated the 
latter phenomenon in Selachians by determining the segment 
in which in successive stages the biliary duct was found; 
the latter was shown to recede into segments situated 
progressively further backwards. In Pristiurus, embryonic 
1) The last occipital nerve of Acanthias, however, has already been 
designated by FüRBRINGER as a, in forms like Scyllium only it IS 2 
