GENERAL DEVELOPMENT. 



On the ventral aspect of the head, and immediately in front of 

 the mouth, is placed the olfactory pit (fig. 43, ol). It is from the 

 first unpaired, and in just-hatched larvae simply forms a shallow groove 

 of thickened epiblast at the base of the front of the brain. By the 

 stage represented in fig. 43 the ventral part of the original groove is 

 prolonged into a pit, extending backwards beneath the brain nearly 

 up to the infundibnlum. 



On the side of the head, nearly on a level with the front end of 

 the notochord, is placed the eye (fig. 43, op). It is constituted (figs. 

 45 and 46) of a very shallow optic cup with a thick outer (retinal) 

 layer, and a thin inner choroid layer. In contact with the retinal layer 

 is placed tne lens. The latter is formed as an invagination of the 

 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, an.r.), 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 thalamen- 

 cephalon (th] and an undivided 

 cerebral rudiment (c.A). To the 

 roof of the thalamencephalon is attached a flattened sack (pn] which 

 is probably the pineal gland. The floor is prolonged into an infundi- 

 bulum (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 hiiid- 

 brain (md) ; the front border of the roof of which is thickened to 

 form the cerebellum (c&). The medulla passes without any marked 

 line of demarcation 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 throughout. The notochord (ch] 

 is continued forwards in the head to the hinder border of the infun- 

 dibulum. 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 myotomes, 



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

 vi i v inadequate in the present state of our knowledge. 



FIG. 44. DIAGRAMMATIC TRANSVERSE SKC- 

 TIONS THROUGH THE BRANCHIAL REGION OF A 



YOUNG LARVA OF PETROMYZON. (From Ge- 



genbaur; after Calberla.) 



</. branchial region of throat. 



