156 OUTLINES OF CHORDATE DEVELOPMENT 



olfactory chamber and in the posterior wall of the internal 

 nares. 



4. The Sense Organs of the Lateral Line 



As mentioned above, the sense organs of the lateral line 

 are derived from the placode of the X cranial nerve, and are 

 innervated by the ramus lateralis of this nerve. When the 

 embryo has elongated to about 4 mm. a small dorso-lateral 

 section of the vagus ganglion separates from the remainder 

 (Fig. 53, A, B) remaining closely in relation with the ecto- 

 dermal placode (Harrison). The placode now begins to 

 elongate posteriorly; the deeper cells multiply rapidly and 

 form a long narrow tongue which pushes along through the 

 epidermis just outside the basement membrane (Fig. 53, (7). 

 Finally, just before hatching, it reaches to the tip of the tail. 

 Differentiation of this cord progresses posteriorly commencing 

 in the older anterior part. The cells become grouped at inter- 

 vals, each group representing the rudiment of a sense organ 

 of the lateral line. In each rudiment a few central cells be- 

 come sensory and are surrounded by a layer of enveloping 

 cells (Fig. 53, D). These groups push up through the epidermal 

 layer to the surface of the body, and the sensory cells develop 

 hair-processes. 



As the placodal rudiment grows posteriorly it is accompanied 

 by outgrowths (axons) from neuroblasts of the dorso-lateral 

 portion of the vagus crest ganglion. These processes lie within 

 the basement membrane of the epidermis, but when the defi- 

 nite sense organs develop the nerve fibers pass up among the 

 sensory cells. The cells forming the medullary sheaths of 

 these fibers appear to wander along the fibers from the vagus 

 ganglion. 



Similar series of integumentary sense organs are formed in 

 definite rows on the head, and dorsally from the lateral line; 

 these are innervated by branches of the VII, IX, and X nerves. 

 The VIII nerve and the auditory organ also belong in this 

 group of sense organs. At metamorphosis they all disappear 

 save the auditory organ and nerve. 



