THE ORGANS OF THE INTERMEDIATE LAYER OR MESEXCHYME. 631 



pounded his segmental theory of the skull, the way has been 

 prepared in many directions, chiefly through emhryological investi- 

 gation, for a better comprehension of the skeleton of the head. 



Investigations which I undertook on the dermal skeleton of 

 Selachians, Ganoids, and Teleosts, as well as on the head-skeleton of 

 Amphibia, showed that the difference between primordial and cover- 

 ing bones is much greater than it was originally assumed to be. 

 For as their development shows, the covering bones are at first 

 structures quite foreign to the axial and head-skeleton, formed at the 

 surface of the body in the skin and mucous membrane. They are 

 parts of a dermal skeleton, which in lower Vertebrates protect the 

 surface of the body as a scaly armor, parts which have entered 

 into union with the superficially located portions of the inner, 

 primordial cartilaginous skeleton. Therefore the covering bones of the 

 lower Vertebrates are often tooth -bearing bony plates, which have 

 originated from a fusion of isolated dental fundaments, a condition 

 which may be regarded for many reasons as the primitive one. 



A further acquisition of broad significance is the discovery of the 

 primitive segments of the head, which we owe to BALFOUR, MILNES 

 MARSHALL, GOETTE, WIJHE, and FRORIEP. 



By it an important point of agreement between head and trunk 

 has been made out. The two body-sacs penetrate even into the 

 head ; here also the two middle germ-layers are separated into a 

 dorsal portion, lying in contact with the chorda and neural tube, 

 which is divided into nine pairs of primitive segments,* and into a 

 ventral portion (see p. 351). 



The head is therefore segmented similarly to the trunk, even at a 

 time when the first traces of the fundament of a vertebral column or 

 a head-skeleton are not yet present. 



Thirdly, the insight into the development of the cranial nerves 

 (BALFOUR, MARSHALL, WIJHE, and others) is important. An agree- 

 ment with the development of the spinal nerves has been established 

 in so far as some cranial nerves have a dorsal origin from a neural 

 crest, like the sensory roots of spinal nerves, while others grow out 

 ventrally from the brain-vesicles like anterior roots. 



Finally, I would mention as a step in advance, which is not with- 

 out significance for the interpretation of the head-skeleton, the 

 altered conception of the meaning of the primitive segments which 

 embryological evidence has compelled us to form. 



The primitive segments are the real fundaments of the musculature 

 * [See footnote p. 458.] 



