with the interval between the scleromeres or with the intervertebral 

 fissure of von Ebner. Thoiigli, as far as I know, the question as 

 such has never been put, it can however be answered with certainty 

 from the literature. It has indeed always been found ') (c.f. the well 

 known investigations of Froriep, Weiss, Gaupp, Bakdeen andolhers) 

 and my own investigations on shcep-enibryones confiim this in every 

 resi)ect, that the cranioverlebral interval coincides with a segment 

 or scleromere interval, and that the most caudal part of the cranium 

 is always formed by a caudal segment-half. This can be most easily 

 ascertained by paying attention to the nerves. The nerve running 

 in the cranial half of the scleromere, the caudal half of which forms 

 the most caudal part of tiie cranium, forms with the two nerves 

 of the two scleromeres lying in a cranial direction from it, the I'oots 

 of the N. hypoglossus; the nerve in the cranial half of tlie next 

 following segment in caudal direction, is the free I'*' cervical-nerve 

 running outside the cranium {c.f. Fig. 3). The caudal half of the 

 last segment belonging to the craiunm is always strongly developed 

 and by its intensive colour disliiicliy to be distinguished from the 

 weakly tinged cranial half of tiie in caudal direction next following 

 segment belonging to the region of the vertebral column, in which 

 cranial half always the first cervical-nerve is found. 



If now we pay careful attention to the fact ascertained by obser- 

 vation, that the cranio-vertebral interval is an intersegmental one, 

 it appears immediately that necessarily, in consequence of the pi-ocess 

 of the resegmentation of the vertebral column, one segment-half 

 remains between the first cervii'al vertebra and the occipital bone. 

 An illustration of this otfers fig. 3. 



We see in it as in Fig. 2 a^ representation of a row of segments, 

 in vvhich axially the chorda extends itself, and which in a lateral 

 direction are linuted by the myotomes somewhat further differentiated 

 in comparison with Fig. 2, from which the myocoel has disappeared. 

 Here the caudal half is likewise striped ; in the cranial segment-half 

 the spinal-nerve (?i..y.) is indicated whilst the intersegmental vessels 

 {a.i.s.) limit the segments. The line A.B. represents the cranioverte- 

 bral interval situated intersegmentally. 



In the process of the resegmentation described above, the vertebrae 

 are formed from the segments in such a way that the caudal half 

 of each segment fuses with the cranial half of tlie next following 

 segment in a caudal direction. So e.g. the caudal half of the fourth 

 segment {S. IV) will fuse with the cranial half of the fifth segment 



') These statements only regard A'liniotes. 



14* 



