144 CHARLES EUGENE JOHNSON 



sion of its cavity, and greatly exceeds in size that attained by 

 either of the other two. From the walls of this vesicle the re- 

 maining muscles of the eye, the so-called oculomotor group, are 

 destined to arise. Since in the next following stage the first 

 traces of these muscles appear, the further history of the head 

 somites will be followed under the section dealing with the devel- 

 opment of the eye muscles. In this process of development the 

 abducent muscle anlage is the first to appear, followed succes- 

 sively by the anlages of the superior oblique muscle and the 

 oculomotor group. For this reason it is thought best to treat 

 the groups in this order. 



THE DEVELOPMENT OF THE EYE MUSCLES 



9-mm. embryo; sagittal series; reconstruction: figure 20 



In this specimen the epiphysis and paraphysis are both promi- 

 nent outgrowths. The nasociliary branch of the ophthalmic 

 nerve extends forward over the dorso-median surface of the 

 optic cup. The short mandibular nerve {man. v.) indents the 

 upper portion of the maxillo-mandibular muscle-mass on its 

 lateral surface but the maxillary nerve has not yet appeared. 

 In the second and third branchial arches condensations of the 

 mesenchyma have appeared, forming the muscle anlages (mus. 

 2, 3) of these arches, and the facial and the glossopharyngeal 

 nerves extend ventrally along their respective borders. 



The abducent muscle-mass lies in the horizontal plane, just 

 mediad of the maxillo-mandibular trunk of the N. trigeminus. 

 Viewed from the dorsal side, as seen in the model, it is a stout 

 mass in which the two divisions before described are well shown. 

 The more rounded posterior portion (retr. oc.) lies nearer the 

 midline, and the anterior part {rect. lat.), represented by an elon- 

 gate narrower extension from the lateral or outer half of the 

 former, reaches considerably beyond the maxillo-mandibular 

 nerve trunk. In sagittal sections the cephalic end of the anterior 

 division presents a heaped-up disposition of its cells, which dif- 

 ferentiates it easily from the rest of this division, in which the 

 spindle-shaped cells have a uniform parallel course. This differ- 



