THE ORGANS OP THE INTERMEDIATE LAYER OR MESENCHYME. 649 



the lateral processes. In the neck-region there is retained, 

 between the transverse process and the rudiment of the 

 rib, the foramen transversarium for the vertebral artery. 



(3) Atlas and epistropheus [axis] assume special forms, owing to 



the fact that the body of the atlas remains separate from 

 the fundaments of its arch, and unites with the body of 

 the axis to form its odontoid process. (Separate centre 

 of ossification in the odontoid process.) 



(4) The sacrum results from the fusion of five vertebrae and the 



sacral ribs belonging to them. The latter by their fusion 

 produce the so-called massse laterales, which bear the 

 articular surfaces for the ilium. 



B. The Head-Skeleton. 



16. The skull, like the vertebral column, passes through three 

 morphological conditions, which are designated as membranous and 

 as cartilaginous primordial cranium and as bony cranial capsule. 



17. The membranous primordial cranium consists of 



(1) The anterior end of the chorda, which extends to the anterior 



margin of the mid-brain vesicle, and 



(2) A connective-tissue layer, which surrounds the chorda as 



skeletogenous layer, and also furnishes a membranous 

 investment around the five brain-vesicles. 



18. The cartilaginous primordial cranium arises by a histological 

 metamorphosis of the membranous one. 



(1) At the sides of the chorda there are first formed two car- 



tilaginous rods, the two parachordals, which soon grow 

 around the chorda both above and below, and become 

 united into a single cartilaginous plate. 



(2) In front of the parachordals RATHKE'S trabeculae cranii 



make their appearance ; their posterior ends soon unite 

 with the parachordal cartilages, their anterior ends 

 become enlarged and by fusing with each other produce 

 the ethmoid plate ; in the middle they remain for a long 

 time separate and embrace the hypophysis (region of 

 sella turcica). 



(3) From the cartilaginous base of the cranium thus produced, 



the process of chondrification, as in the development of 

 the vertebral column, first extends into the lateral walls, 

 and at last into the roof of the membranous primordial 

 cranium, partly enclosing the higher sensory organs. 



