No. i.] MORPHOLOGY OF VERTEBRATE HEAD. 85 



distinguished by their different positions. In both, the cells of 

 the inner wall are becoming muscular, and in both these cells 

 are migrating to the centre of the cavity. Sec. 4 shows that 

 the walls of the premandibular cavity are but one cell in thick- 

 ness, except where they are about to be continued into the pro- 

 cess which forms the inferior oblique muscle {inf. obi). 



A break (Fig. 3, b, PI. IV) has appeared in the inner wall 

 of the mandibular cavity, where, posterior to the rudimentary 

 muscle (mus. e.) of sees. 10 and 4, the wall was seen in sec. 10, 

 PL VI, to be very thin. The division thus begun ultimately 

 separates the cavity into two parts, of which the posterior and 

 central part belongs to the mandible, the anterior and dorsal 

 part to the region of the eye. I have not traced the history of 

 the mandibular muscles, but may mention that a small muscle 

 arises just posterior to the break (Fig. 3, b), which I believe 

 to be permanent. It is found later in the neighborhood of the 

 inferior oblique eye-muscle. The muscle mus. e. of Fig. 2, has 

 become greatly enlarged in the stage represented by Fig. 3. 

 It continues the line of the external rectus muscle which is 

 now forming from the walls of the third head cavity. These 

 two muscles are so intimately connected that for some time I 

 believed the mandibular cavity to take part with the third in 

 the formation of the external rectus muscle. The walls of the 

 anterior prolongation of the mandibular cavity have begun their 

 metamorphosis into the superior oblique muscle. Both outer 

 and inner walls take part in the change, but the inner wall is 

 the more active. Their relative thickness does not vary essen- 

 tially from that represented in sec. 10, PI. VI. Cells from the 

 posterior wall of the third head cavity are fast dropping off into 

 the general mesoderm. It will be noticed that the direction of 

 the long axis of this cavity is changing. The ultimate change 

 is one of 180 , so that the anterior limit of the original cavity 

 forms the posterior or distal attachment of the external rectus 

 muscle. 



Between the stages represented in Figs. 3 and 4 the length 

 of the embryo has increased from 16 to 22 mm. In Fig. 4 the 

 outline of the anterior head cavity (seen through the walls of 

 the premandibular) appears much as in Fig. 3, but the cavity no 

 longer exists. Its place is occupied by a compact mass of cells. 

 That part of the premandibular cavity which was cut off to 



