86 PL ATT. [Vol. V. 



form the inferior oblique muscle has also been filled by the 

 ingrowth of muscle cells, and the solid muscle thus formed has 

 begun to extend in a ventral direction (inf. obi., Fig. 4). Sec- 

 tions through this muscle and through the muscle formed from 

 the walls of the anterior head cavity still closely resemble one 

 another. The dorsal wall of the premandibular cavity has now 

 become thickened to form posteriorly the rudiment of the infe- 

 rior rectus muscle (inf. rec), anteriorly the rudiments of the 

 superior (sup. rec.) and the internal (int. rec.) recti muscles, which 

 are separated from one another by a slight depression in the 

 wall of the cavity. The internal rectus is the more median 

 of the two anterior muscles. The walls of the premandibular 

 cavity are elsewhere thin. The visceral part of the mandibular 

 cavity has now become entirely severed from the remainder of 

 the cavity, and is omitted in the reconstruction (Fig. 4). The 

 dorsal part of the cavity is now represented by a solid mass of 

 cells, forming anteriorly the superior oblique muscle, posteriorly 

 the muscle marked mus. e. The connection between these two 

 muscles has been almost obliterated in the region where the 

 ramus ophthalmicus profundus trigemini crosses the mandibular 

 cavity in passing from the Gasserian ganglion, which is situated 

 lateral to the cavity, to the ciliary ganglion, which lies median 

 to the cavity. The third head cavity has become completely 

 filled by muscle cells that have migrated to the centre of the 

 cavity from its inner wall, as shown in sec. 5, PI. V. This section 

 also shows the peculiar relation existing between the external 

 rectus muscle, now forming in the third head cavity, and the 

 mandibular muscle, mus. e. If this cross-section be compared 

 with a similar section through an older embryo, represented in 

 sec. 6, it will be seen that were the cells of the mandibular 

 muscle (mus. e.) to fuse with those of the third head cavity 

 (ext. rec) at the stage represented by sec. 5, the resulting mus- 

 cle would closely resemble in shape the external rectus of sec. 6. 

 I am convinced, however, that such a fusion does not take place, 

 for the limiting wall of the third head cavity can be traced until 

 the muscle here formed comes to occupy the entire place once 

 occupied (sec. 5) by the cells of the two muscles (ext. rec. and 

 mus. e.). The cells of the mandibular muscle (mus. e.) gradually 

 yield their place to those of the third head cavity and are ulti- 

 mately lost in the general mesoderm. Thus a muscle, the rudi- 



