60 SYDNEY EVANS JOHNSON 



able distance posteriorly. The 40 mm. specimen was too badly 

 shrunken to afford material for histological study. 



Forty-five millimeter stage. In a 45 mm. specimen the trunk 

 canal was closed for a short distance posterior to the pectoral 

 fin. Figm-e 66 shows a transverse section of the canal anterior 

 to the pectoral fin. Here the canal has just closed and has not 

 yet separated from the epidermis. Figure 67 shows a trans- 

 verse section a little posterior to the pectoral fin, where the 

 canal lacks only a little of being closed. Considerably farther 

 back the canal is represented by a furrow (fig. 68) which becomes 

 shallower posteriorly. The formation of the canal resembles in 

 its general features the closure of the neural groove to form the 

 neural tube. 



Fifty-two millimeter stage. In this specimen the canals of 

 the head are all closed, although the surface openings of the 

 infraorbital are still rather large and somewhat elongated. 



The trunk canal as shown in figure 69 is closed nearly to the 

 second dorsal fin. Figure 70 is a cross section of the canal a 

 little farther forward where it has separated from the epidermis. 

 Here the lumen is very small and the condition might easily 

 be mistaken for a solid cord of cells. Posterior to the closed 

 portion it is represented by an open furrow. 



The separated organs of the accessory and dorsal series have 

 reached their permanent number (dorsal series, 11 or 12; acces- 

 sory, about 41) and the former have begun to sink slightly be- 

 low the surface of the ectoderm. These structures, however, 

 do not become enclosed within canals. 



Sixty-seven millimeter stage. In this specimen (figs. 71, 

 72) the canal is tubular for a somewhat greater distance poste- 

 riorly. In the region of the first dorsal fin the lumen of the canal 

 has become enlarged and its epithelial walls are relatively thin- 

 Fig. 65 Ventral view of the same specimen as represented by figure 64, show- 

 ing distribution of sensory canals on the inferior surface of the head. 



Figs. 66, 67, 68 Transverse sections of tlie lateral canal and cord at different 

 levels of the body. Figure 66, anterior to the pectoral fin; figure 67, posterior 

 to pectoral fin; and 68, posterior to second dorsal fin. All three figures from a 

 45 mm. specimen. Squalus. X 492. 



