193 The Development of the Eye Muscles in Acanthias 



The thickenings of the epithelial walls which give rise to the four 

 muscles^ occur in general on the dorso-median walls. The first thicken- 

 ing appears on the more ventral end of the somite. This soon becomes 

 a large outpocketing with thick walls (Fig. 3, I. Ohl.). It is the anlage 

 of the muscle obliquus inferior. This therefore is, as Hoffmann pointed 

 out, the first of the oculomotor muscles to be differentiated. 



The next thickening to appear is located very near the above- 

 mentioned anlage of the inferior oblique. It gives rise to the muscle 

 rectus inferior (Fig. 3, Inf. Bee). At a later stage thickeniugs appear 

 on the more dorsal end of the somite. These do not become well 

 marked and differentiated from one another until a somewhat late stage, 

 about 20-22 mm. The more dorsal of these thickenings forms the 

 muscle rectus superior; the more ventral, the muscle rectus interior. 



It will be seen from the above that the muscles arise in two pairs, 

 one at either end of the somite. 



The outpocketing which is to form the inferior oblique soon becomes 

 constricted off froin the somite. At a 19 mm. stage its lumen has 

 nearly disappeared, and the muscle has assumed an elongate form 

 (Fig. 3). The direction of the principal axis as well as the direction 

 of its muscle fibres is longitudinal. In the adult the direction is also 

 nearly longitudinal, but it will be seen from the series of reconstruc- 

 tions (Figs. 4, 6, 7, 8), that the originally anterior end has become 

 posterior; i. e., the direction of the muscle is nearly reversed. This 

 transformation is brought about by a revolution of the posterior end 

 about the anterior end as a centre, combined with a general ventral 

 shifting of the muscle. The adult condition is approximately reached 

 at a 33 mm. stage (Fig. 8). 



The thickening, which is to form the inferior rectus, and belonging 

 to the same pair as the inferior oblique, at first extends parallel to 

 that muscle and therefore in a longitudinal direction (Fig. 4, Inf. Eec). 

 At a 26 mm. stage it has turned through approximately a right angle, 

 and runs in a general dorso-ventral direction (Figs. 6, 7, Inf. Rec). 



The thickenings which arise in the more anterior and dorsal end of 

 the somite, and which give rise to the superior and internal recti, have 

 only become clearly differentiated at a 25 mm. stage (Fig. 7). The 

 internal rectus retains its nearly longitudinal direction; the superior 

 rectus describes approximately a right angle about its posterior end. 

 At a 33 mm. stage (Fig. 8) it has approximately reached its adult dorso- 

 ventral direction. Except where the muscle thickenings have been 

 formed, the walls of the somite retain their single layered epithelial 

 character until about a 27-30 mm. stage, when they become converted 

 into loose mesenchyme and the outline of the somite is lost. 



