184 ARTHUR E. SHIPLEY, 



the embryo they contain yolk granules, which are gradually 

 absorbed. In the tail region these mesoblastic somites con- 

 tinue to be segmented off from the primitive streak till five 

 or six days after the larva is hatched. 



In transverse sections the mesoblastic somites appear trian- 

 gular, having a median side against the nervous system and 

 notochord, an external one against the epididymis and a ventral 

 one. Besides these there are the anterior and posterior sides. 

 The cells composing all these, except those of the external 

 layer, develope into longitudinal muscles. Whilst this is taking 

 place the dorsal surface of the embryo has become raised above 

 the general level, so that the embryo in section is no longer 

 round but pear-shaped. 



As Stannius, Grenacher, and Langerhans have 

 shown, the muscles of the Lamprey fall into two groups, which 

 differ in structure as well as in their disposition. The first of 

 these form the myomeres, and are derived directly from the 

 mesoblastic somites ; the second comprise the muscles of the 

 eye, those belonging to the respiratory system, and those con- 

 nected with the upper and lower lip and mouth generally. 

 These seem to arise exclusively from the ventral unsegmented 

 parts of the mesoblast, and perhaps, in some eases, from wan- 

 dering mesoblast cells. The muscles of the heart resemble the 

 latter in many points. 



Each myomere in the Lamprey or Ammoccete consists of a 

 number of plates of muscle-substance, lying one on the top of 

 another. Each plate is flat, and more less square in outline. 

 It is bounded anteriorly and posteriorly by the myotomes ex- 

 ternally by a connective-tissue layer closely connected with the 

 skin, and internally by a similar layer. Above and below, or 

 dorsally and ventrally, it is in contact with a similar muscle- 

 plate. In some myomeres which have become modified, such 

 as the anterior one which extends far forward over the ear, the 

 shape of the muscle-plate has lost its square outline and be- 

 come oblong, but in one of the myomeres of the trunk they 

 are almost square in longitudinal section. 

 Prom the above description it will be seen that each muscle- 



