ORIGI N AND STRUCTURE OF THE HEAD 77 



contrary on the outer side. After FRORlEP (1901), who, as 

 we have seen, is inclined to emphasize the contrast between 

 the "praespinal" or "cerebral" part of the head and the 

 rest of the body, we have to do here with two different 

 neural crests, which in the occipital region run along each 

 other for some distance. DOHRN (1902) however does not 

 admit this contrast and GUTHKE (1906) finds that the 

 spinal ganglion crest behind the last gill- slit crosses the 

 eighth segment, thus passing from the inner side of the 

 somites to the outer side, where the head ganglia are found. 

 According to ZlEGLER (1908) the lateral situation of the 

 head ganglia stands "wahrscheinlich in Beziehung zu der 

 Placodenbildung und ist demnach die Folgeder Ausbildung 

 der eigenartigen Sinnesorgane, als deren palingenetische 

 Reste jetzt die Placoden auftreten". Finally Gast (1909, p. 

 375) believes, that a double ridge is found not only in the 

 region where cephalic and trunk neural crest, according 

 to FRORIEP, run for some distance parallel to each other, 

 but that this double nature is characteristic of the whole 

 head and trunk. According to the conclusions arrived at 

 by us regarding the relations between spinal and lateral 

 ganglia, the lateral situation, if ZlEGLER's view were right, 

 would have been the primitive one also for the spinal 

 ganglia, and the present situation, on the inner side of the 

 somites, ought to be considered as secondarily acquired 

 after the loss of the segmental communications of the 

 spinal with the lateral ganglia. A last trace of this might 

 be found in Gast's observation of the double nature 

 of the trunk neural crest. But here we may stop our 

 speculations in this direction, in which we may, perhaps, 

 have gone too far already. 



Eye-muscle nerves. — Finally the oculomotorius, trochlearis 

 and abducens have been compared by VAN Wyhe (1882) 

 to the ventral roots in the trunk region. The oculo- 

 motorius is considered as the ventral root of the first or 

 praemandibular segment and innervates the eye-muscles 

 derived from it, the trochlearis as the ventral root belong- 

 ing to the second or mandibular segment, while the 

 abducens in counted as belonging to the third or hyoid 

 segment. 



We will revert later to the question whether a ventral 

 root belongs to the following segments (ventral vagus roots 

 of GegENBAUR, hypoglossus in Amniotes!). Thus the number 



