Arthur B. Lamb 195 



visceral part. At a 13 mm. stage the lumen of the one part is com- 

 pletely separated from that of the other. Until a late stage, however 

 (24 mm.), the two parts are connected by strands of mesenchymatous 

 tissue (Ifig. 5, MS.). 



For the sake of clearness I will treat the further development of 

 dorsal and visceral parts separately. 



Dorsal Part, or the Myotome Proper. — At a fourteen mm. stage a large 

 outpocketing with slightly thickened walls is apparent at the anterior 

 end of this part of the somite. This outpocketing is the anlage of the 

 muscle obliquus superior. At the base of this outpocketing, on the 

 median side of the somite, a thickening of the epithelium is evident. 

 This is the anlage of a rudimentary muscle first mentioned by Miss 

 Piatt, and spoken of by her as " Muscle E." The walls of the remainder 

 of the dorsal part are very thin. 



Sixteen mm. stage (Kg. 3). The outpocketing giving rise to the 

 superior oblique has become very thick-walled. " Muscle E '' is well 

 developed. The direction of its principal axis, as well as of its fibres, 

 is longitudinal. It is not straight, however, but its anterior end is 

 curved outwards. 



From this stage on, those parts of the walls not forming muscles 

 rapidly degenerate, and the lumen of the somite is usurped by mesen- 

 chymatous tissue. A contracted lumen, however, persists until a late 

 stage in the anlage of the superior oblique muscle. The general direc- 

 tion of this muscle and of the fibres which are now present in it, is 

 longitudinal. The whole muscle migrates forward, the posterior end 

 becoming attached, while the anterior end moves ventrally. The mus- 

 cle consequently comes to extend in a dorso-ventral direction (Figs. 4, 

 5, 8, S. Ohl.). At a 19 mm. stage (Fig. 4) it is still longitudinal and 

 still retains connection with " Muscle E " by a strand of connective 

 tissue. This latter muscle has now reached its maximum development. 

 The anterior end curves not only outward but upward as well, so that 

 the direction of the muscle is approximately dorso-ventral. From now 

 on this muscle undergoes degeneration. At a 26 mm. stage scarcely a 

 trace of it remains. 



Visceral Part. — At a 12 mm. stage the walls of this part, except 

 "where they are continuous dorsally with the somite proper, are closely 

 compressed right and left. At the ventral end the walls are very thick, 

 and constitute the anlage of the muscle adductor maxillse. 



At a 14 mm. stage an outpocketing appears at the dorsal end. This 

 is the anlage of a rudimentary muscle which Miss Piatt, gi, first recog- 

 nized. Hofl'mann, 96, considered this muscle identical with that de- 

 15 



