v CHONDKOCEANIUM 317 



inconsiderable tectum synoticum, which originates as a pair of 

 at first separate cartilaginous rods (Chick 21 mm.). These very 

 soon become continuous with one another and with the auditory 

 capsule. 



Development of Chondrocranium in General. — The three 

 examples of chondrocranial development which have been dealt with 

 will suffice to give a general idea of the process with its variations. 

 A survey of the known facts in Vertebrates generally shows that 

 the first rudiments of the chondrocranium consist of paired elongated 

 pieces of cartilage (preceded by prechondral tissue) lying on each 

 side of the mesial plane and on the morphologically ventral side of 

 the brain. These rudiments are divisible into a (para)chordal 

 portion lying at the side of the notochord, and a prechordal portion 

 lying anterior to this. A break in the continuity of the cartilage 

 frequently occurs somewhere about the limit between these regions 

 and this had led to the regarding of the portions so separated — 

 trabecula in front and parachordal behind — as being fundamentally 

 distinct morphological elements. As a matter of fact the break, 

 when it does occur, appears to vary in position : thus in Petromyzon 

 the "trabeculae" extend back for some distance beyond the tip of 

 the notochord, so that their hinder parts are parachordal in position. 

 In many cases the break is visible only for a very short period, 

 while in others (Zepidosiren) there is complete continuity between 

 trabecula and parachordal. On the whole it appears justifiable in 

 the present state of our knowledge to regard the break in continuity 

 between trabecula and parachordal not as marking a demarcation 

 between two originally distinct morphological elements but rather 

 as a secondary solution of continuity correlated with exaggerated 

 cerebral (mesencephalic) flexure. 



The parachordal cartilage in the case of the Elasmobranchs 

 passed backwards by perfectly insensible gradations into the cartilage 

 of the vertebral column. In that portion (occipital region) which 

 lies between the hinder limit of the definitive cranium and the 

 vagus nerve there appear for a time evidences of segmentation, 

 corresponding with that of the vertebral column, and it is therefore 

 justifiable to regard this portion of the parachordal cartilage as 

 representing a region of fused vertebrae. In the anterior or mesotic 

 portion there are no visible metameric swellings but, as the relations 

 to the notochord are otherwise identical, it is difficult to refuse a 

 homology in this case which is granted in the case of the hinder 

 portion. Here again, then, we should be inclined to regard the 

 distinction between the mesotic and the occipital portions of the 

 parachordal as merely a secondary differentiation in what was once 

 a continuous structure or series of structures : in other words we 

 should regard the whole of the parachordal region of the cranium as 

 representing a modified portion of vertebral column which has been 

 absorbed into the cranium. 



The foundations then of the vertebrate chondrocranium are laid 



