No. 482] M ERISTIC HOMOLOGIES IN VERTEBRATES 107 



out the mechanism involved and may be used to reduce all to a 

 common rule. 



In the same way the assumption that there are similar buddinji; 

 zones at various points in the vertebrate body will explain the 

 various conditions outUned in the statement of the problem. In 

 the vertebrates there is a continuous addition of new somites at 

 the posterior end of the body as in the arthropods and annelids, 

 implying the existence of the equivalents of teloblasts at the pos- 

 terior end. The assumption of budding zones at other points 

 will explain the other features noted. Such a zone in the occipital 

 region will allow us to explain the difference in the number of 

 cranial nerves in the mammals and in the Ichthyopsida and yet 

 allow us to accept the homology of the occipital bones throughout 

 the vertebrate series. The additional nerves are thus to be re- 

 garded not as transferred from the neck but as new or intercalated 

 structures. In the same way we may explain the varying number 

 of vertebrae in the different regions and allow at least one of the 

 pelvic vertebrae to be regarded as a fixed point and may be relieved 

 of any assumption of a shifting of the uinlles. It will also (>\|)laiii 

 many anomalies such as the attacluneiit of the two halves of tin- 

 pelvis to different vertebra^ and the incrcast'd nuiiiher of limibar 

 or thoracic vertebra^ in man. 



This is to be regarded solely as an hypothesis. So far as I am 



fact it is extremely prol>abIe that there is no >wh well .lefined /one 

 as is found in the band of teloblasts of the crnstacean. It is to be 

 regarded rather as a series of assiunptions. Uased in part upon 

 analogies, which, if true, would explain the questions with which 

 the present note began. The hypothesis is presented as a sug- 

 gestion to stimulate investigation and criticism upon an interesting 

 and difficult subject. 



