84 PLATT. [Vol. V. 



through a horizontal plane in the line sec. 10, Fig. 2, PL VI, 

 and represents the mandibular cavity of the opposite side of the 

 head from that seen in the reconstruction. From the same 

 section (sec. 10) it is also seen that the inner wall of the body 

 of the mandibular cavity has become very thin, anticipating its 

 absorption into the general mesoderm. This enlarged part of 

 the cavity is followed by a region where the walls are closely 

 compressed, and this in turn is followed by the rudiment of the 

 large mandibular muscle, " Kaumnskel" (inns. 2 ). The third 

 head cavity (3) has become elongated in an antero-posterior 

 direction, and its posterior wall has opened to the general meso- 

 derm. The walls of these cavities are now developing muscle 

 cells in three places; namely, in the ventral part of the man- 

 dibular cavity to form the " Kaitmnskcl" above mentioned, in 

 the inner wall at the base of the anterior prolongation of the 

 mandibular cavity (sec. 10, mus. e, PI. VI), and in the inner wall 

 of the anterior head cavity (a). The oculomotor (III) and ab- 

 ducent (VI) nerves have appeared, and are distributed to the 

 walls of the premandibular and third head cavities respectively. 

 In Fig. 3, PI. IV, the point of view has changed. The head 

 cavities are now seen from the median plane of the embryo. 

 I thought it well to picture the primary cavities as they would 

 appear if looked at from the side of the head, but I also wished 

 to project the eye-muscles upon the eyeball, and it consequently 

 became necessary to change the point of view at some stage. 

 I have chosen to do so here. Because of this change, the 

 anterior head cavity (a) is represented as if seen through the 

 walls of the premandibular cavity. From sec. 4, PI. V, which 

 passes through the line sec. 4, Fig. 3, PL IV, it will be seen that 

 many cells from the median wall of the anterior cavity (a) have 

 migrated into the centre of the cavity, while the cells bounding 

 the inner wall above and below have the elongated contour of 

 muscle cells. The size and shape of the anterior cavity is little 

 changed. The premandibular cavity, however, has grown greatly. 

 The posterior constriction (1 i.e.) which appeared in Fig. 2 has 

 increased, and the walls of the portion of the cavity thus cut 

 off are fast developing muscle cells. Cross or horizontal sec- 

 tions through an embryo at this stage show the portion con- 

 stricted from the premandibular cavity to resemble so closely 

 the anterior head cavity (a) that the two cavities are only to be 



