58 SYDNEY EVANS JOHNSON 



pratemporal commissure, the two arms of which have not yet 

 grown together, and the infraorbitals, which have not yet 

 reached their anterior Umit (figs. 64, 65). 



The lateral cord has united with the supraorbital and infraor- 

 bital canals, anteriorly, and at this stage, which precedes the 

 formation of the lateral canal, it passes to the tip of the caudal 

 fin. The sensory cord presents the appearance of a minute 

 chain of beads lying at the bottom of a shallow furrow. The 

 differentiation of the sense organs is more advanced in the an- 

 terior part of the sensory cord than in the posterior part. 



The sensory canal system is now practically mapped out and, 

 although great changes occur before it reaches the adult condi- 

 tion, these changes are of a secondary nature, such as changes in 

 relations and closure of the canals. If the system were arrested 

 in its development at this stage, or a little earlier, it would be 

 comparable in a rough way to the lateral sense organs found in 

 certain amphibia, although in the latter the sensory epithelium 

 is much less abundant. 



b. Later stages of developrnent — 40 to 72 mm. In the 36 mm. 

 stage it was seen that the distribution of the sensory cords is 

 practically completed, and the nerve connections apparently 

 all established. Cellular differentiation of the sensory cords is 

 also well advanced. Some changes in relations occur and sur- 

 face tubules become elongated as growth proceeds but the im- 

 portant change which follows is the invagination of the sensory 

 thickenings of the trunk and the gradual enclosure of the main 

 lines of sense organs within epithelial canals. These changes 

 are here described as they appear in embryos of 40, 45, 52, 67, 

 and 72 rmn. in length. 



Forty millimeter stage. The head canals which were well on 

 their way towards involution in the 36 mm. stage are now almost 

 completely closed. The pores of the tubules are much smaller 

 than they were and as a result the canals are less conspicuous in 

 surface views. 



The gi'oove of the lateral line is closed for a short distance in 

 its anterior region and is represented by a furrow for a consider- 



