19? 



the forming of the vertebra? This depends npon the species of 

 animal in question. Willi some animals we see that the originally 

 caudal half and the originalij cranial half have an equal part in 

 the forming of the vertebra. With most higher Amniotes and like- 

 wise with man we see however that, at least as regards the ver- 

 tebral arch, the caudal segmenthalf becomes predominant, whilst the 

 cranial one, partly because the spinal-nerve and the spinalganglion 

 belonging to it always lie in it, gets more into the background. It 

 is not my intention to enter into further parliculais about the share 

 that the two segmenthalves have in the forming of the vertebra. The 

 statements of the divers investigators diverge, which must be partly 

 attributed to the certaiidy very great dilïiculties of the investigation, 

 partly to the fact mentioned already above, that the relations with 

 the different species of animals are not the same in this respect. 

 I will only emphatically point out, that in what way the segment- 

 halves may behave in definite cases in the forming of the vertebra, 

 they naturally possess a complete potency, in such a measure that 

 from each of the two halves under special circumstances a complete 

 vertebi-a can be formed. A proof of this are the so called em- 

 bolomere or )-hachitome vertebrae, which occur frequently with 

 Anamnia, but are likewise found with Amniotes, which was first 

 shown by Goethe with Lacerta viridis, afterwards by Manner with 

 Angius and by Schauinsland with Sphenodon, Castor fiber and 

 Cetaceae. 



After this very short explanation of what is essential in the meta- 

 mery of the cranium and the re-segmentation of the vertebral column 

 we shall examine, to what consecjuence these two dogmas lead in 

 the ontogeny of the craiiio-vertebral region. 



If the doctrine of the metamery of the cranium according to 

 Froriep and the later investigators is correct, and for the present 

 there is no reason to doubt of it, then we must represent to ourselves 

 the region of the spinal part of the cranium (the praespinal part 

 can, as falling beyond the cranio-vertebral region, remain out of 

 consideration) and of the vertebral column in a very young stage 

 of embryonal development, as an uninterrupted row of anatomically 

 (not morphologically) equivalent scleromeres, as is represented schema- 

 tically in Fig. 2. 



Axially the chorda(f/i.) extends, through these scleromeres, the 

 cranial and caudal boundaries of which are indicated by the arteriiae 

 intersegmentales {a.i.s. interprotovertebrales). Laterally from the scle- 

 romeres one sees the myotome belonging to the connected segment 



14 



Proceedings Royal Acad. AiBsterdam. Vol. XV 111. 



