CYCLOSTOMATA. 



93 



skin ; to which it is still attached in the just-hatched larva (fig. 

 45). The eye only differs at this stage from that of other 

 Vertebrata in its extraordinarily small size, and the rudimentary 

 character of its constituent parts. 



The auditory sack is a large vesicle (fig. 43, au.v}, placed at 

 the side of the brain opposite the first persistent branchial 

 pouch. 



The brain is formed of the usual vertebrate parts 1 , but is 

 characterized by the very slight cranial flexure. The fore-brain 

 consists (fig. 43) of a thalamencephalon (///) and an undivided 

 cerebral rudiment (clt). To the roof 

 of the thalamencephalon is attached 

 a flattened sack (pri) which is prob- 

 ably the pineal gland. The floor is 

 prolonged into an infundibulum (in} 

 which contains a prolongation of the 

 third ventricle. The lateral walls of 

 the cerebral rudiment are much 

 thickened. 



Behind the thalamencephalon 

 follows the mid-brain (mb}> the sides 

 of which form the optic lobes, and 

 behind this again the hind-brain 

 (md} ; the front border of the roof of 

 which is thickened to form the cere- 

 bellum (cb}. The medulla passes 

 without any marked line of demarca- 

 tion into the spinal cord. 



The histological differentiation of the brain has already 

 proceeded to some extent ; and it has in the main the same 

 character as the spinal cord. Before the larva has been hatched 

 very long a lateral investment of white matter is present through- 

 out. The notochord (ck) is continued forwards in the head to 

 the hinder border of the infundibulum. It is slightly flexed 

 anteriorly. 



From the hinder border of the auditory region to the end of 

 the branchial region the mesoblast is dorsally divided into 



1 Max Schultze's statements as to the structure and histology of the brain are very 

 inadequate in the present state of our knowledge. 



4. 



FIG. 45. HORIZONTAL SEC- 

 TION THROUGH THE HEAD OF A 

 JUST-HATCHED LARVA OF PETRO- 

 MYZON SHEWING THE DEVELOP- 

 MENT OF THE LENS OF THE EYE. 



th.c. thalamencephalon ; op.v. 

 optic vesicle ; /. lens of eye ; A.c. 

 head cavity. 



