290 George L. Streeter 



sensory nerves. The general cutaneous nerves supplying the whole 

 surface of the body compose the bulk of this group, and they are repre- 

 sented in the head by fibres belonging to the trigeminal, ninth and 

 tenth nerves. In addition there are in the head region special somatic 

 sensory nerves. The union of nerve and integument has resulted in 

 these cases in the formation of special sense organs, that is the olfactory 

 organ, the eye and the ear, composed partly of nerve elements and partly 

 of integument. In this sense the olfactory nerves, the retinal ganglion 

 cells and the acoustic nerve, though differing so widely in their adult 

 morphology, may be considered as analogous in development and function. 



Nn. olfadorii. At the end of the first month the ectodermal olfactory 

 pit is definitely formed and is already in the process of differentiation, 

 evidenced by the increase of body-protoplasm of its epithelial cells. A 

 corresponding pouch is just beginning to form at the olfactory area at 

 the base of the cerebral hemisphere, the anlage of the olfactory bulb. The 

 fibres which later connect the two, nn. olfactorii, cannot yet be made out. 



N. opticus. In case of the optic apparatus the chief contribution on 

 the part of the integument is the lens; and the nerve contribution is 

 the retina. The optic nerve corresponds to the olfactory stalk, and it 

 is formed by the conversion of the optic stalk into a fibre tract connect- 

 ing the retina with the brain. In the embryo studied the optic stalk 

 consists of a thick-walled hollow tube whose cavity still freely communi- 

 cates with the general brain cavity. The lower border is indented by 

 the choroidal fissure which gives it a crescentic outline in cross section. 

 The structure of this stalk is like that of the wall of the hemisphere, 

 consisting of a thick ependymal layer covered in by a thin fibrous mar- 

 ginal zone. This marginal zone later thickens at the expense of the 

 ependymal layer and is converted into a framework through which the 

 nerve fibres from the retina make their way to the brain. At the 

 end of the first month we cannot yet refer to this stalk properly as the 

 optic nerve. 



N. acusticus. This nerve is in a more advanced stage of development 

 than the optic and olfactory nerves. The epithelial part of this appa- 

 ratus, the ear vesicle, is already pinched off from the skin, as is shown 

 in Plate I, and consists of a closed sac consisting of a double pouch 

 having an upper vestibular portion and a lower cochlear portion; at the 

 junction of the two on its median side is attached the ductus endolym- 

 phaticus. 



The acoustic ganglion lies closely against the cephalic and median 

 border of the ear vesicle. Two groups of fibres from the ganglion can 



