No. i.] MORPHOLOGY OF VERTEBRATE HEAD. 83 



(spiracular) cleft alone has broken through. The median part 

 of the premandibular cavity (1) is indicated as if seen through 

 the walls of the mandibular cavity (2). I have designated the 

 new head cavity (a), " anterior," rather than "first," as the nu- 

 merals, 1, 2, 3, have been already assigned to the premandibular, 

 mandibular, and " third " head cavities respectively. The walls 

 of all four cavities are but one cell in thickness, and are as yet 

 undifferentiated, save that the cells forming the wall of the 

 ventral portion of the mandibular cavity are deeper than those 

 found elsewhere, and the walls of this part of the cavity are 

 somewhat irregular, owing probably to the rapid proliferation of 

 cells from the surrounding mesoderm. The " third " head cav- 

 ity (3) occupies the place of the most anterior of the three 

 incomplete divisions of the mesoderm which were found at an 

 earlier stage above the hyoid arch. 



Between the stages represented in Figs. 1 and 2 the em- 

 bryo has increased from 6 to 12 mm. in length. The rela- 

 tive increase in size is shown by the reconstructions, as they 

 are made from camera outlines of a like magnification. The 

 diameter of the last two reconstructions (Figs. 6 and 7), how- 

 ever, has been reduced one-half. In the embryo from which 

 Fig. 2 is reconstructed, the anterior head cavity (a) has in- 

 creased in size, but its shape is little changed. In relation 

 to the eye, the cavity maintains approximately the same posi- 

 tion. In relation to the other head cavities, however, its posi- 

 tion is greatly changed, owing in part to the increased cranial 

 flexure, which has carried the cavity to a position apparently 

 posterior to that originally occupied, and in part to the rapid 

 growth of the lateral portions of the premandibular cavity (1), 

 against the outer walls of which the anterior cavity (a) has now 

 come to lie. The median portion of the premandibular cavity 

 has reached its maximum development, and posteriorly the 

 cavity is becoming constricted, prior to the formation of the 

 inferior oblique muscle. As yet no muscle cells have appeared 

 in its walls. The anterior prolongation of the mandibular cavity 

 (2) now lies in a plane more nearly horizontal, a change due 

 chiefly to the increased cranial flexure, and to the accompany- 

 ing rapid growth of the dorsal region of the head. The inner 

 wall of the anterior prolongation of the mandibular cavity 

 has become concave, as shown in sec. 10, PI. VI, which passes 



