210 ARTHUB B. SHIPLEY. 



bears a knob at its free end (fig. 41). About tbe same time a 

 number of small concretions appear in the ear. These form 

 the numerous spherical otoliths. 



Summary. 



I have now described the structure of the chief organs in 

 my oldest larva, and I propose to conclude this paper by a brief 

 summary of the results obtained. 



In the first place the mesoblast is not completed ventrally by 

 a layer of cells split off from the hypoblastic yolk-cells, as 

 Scott has described. But the ventral mesoblast is formed by 

 the downgrowth of the mesoblastic plates, which ultimataly 

 meet and unite in the ventral middle line. 



The blastopore does not close up, as later observers have 

 maintained, but, as Max Schultze described thirty years ago, 

 it persists as the anus. There is no neurenteric canal, though 

 a solid strand of tissue proceeds back from the alimentary 

 canal and fuses with an indifierentiated mass of cells, into which 

 the nervous system and mesoblast also pass. 



The lumen of the alimentary canal is that of the mesenteron ; 

 it does not become obliterated during larval life. In its anterior 

 end the hypoblast remains in connection with the epiblast at 

 certain points, and here the gill-clefts arise ; between these the 

 mesoblast grows down and forms the gill-bars. The origin of 

 the ciliated ring and the hypopharyngeal groove and hyper- 

 pharyngeal bar are also described, and the ciliated condition 

 of the oesophagus and stomach. 



The " muscle- plates,'^ whose structure is so peculiar in the 

 Lamprey, arise each from a single cell of the mesoblastic 

 somites. This increases in size, slides in between the neigh- 

 bouring cells, and ultimately occupies the whole of the space 

 between two myotomes. Its nucleus divides until each cell 

 contains several nuclei. Striated fibrils then appear and in- 

 creases till the whole " muscle-plate " consists of little else be- 

 sides these fibrils, squeezing between them a few nuclei. These 

 *' muscle-plates " arise from the segmental half of the meso- 

 blast ; the muscles of the gills, lips, and probably of the eye. 



