182 PROCEEDINGS OF THE AMERICAN ACADEMY 



mes. Mesoderm. 



wi ve. Ventral mandibular elevation. 



m X. Maxillary branch of niandibular arch. 



0. Otic vesicle. 



pc. I'ritnary elevation on the liyoid arch. 



s d. The dorsal secondary elevation of tiie hyoid arch. 



6 V. The ventral secondary ilcvation of the hyoid arch. 



vp. Ventral depression of the first cleft. 



EXPLANATION OF FIGURES. 



Fig. 1. Head and neck of an embryo pig G mm. in length. 



Fig. 2. Side view of an embryo 8 mm. long, in the region of the clefts. 



Fig. 3. Same view of an embryo 9 mm. long. 



Fig. 4. Same, of an embryo 10 mm. long. 



Fig. 5. Side view of the first cleft of an embryo 11 mm. long. 



Fig. G. The same region from an embryo 17 mm. long. 



Fig. 7. The incipient auricle at a stage 19 mm. long. 



Fig. 8. The same at a 25 mm. stage. 



Figs. 9-11. Three successive sections tangential to the left side of the neck, 

 cut nearly parallel to the sagittal i)lane, from an cmbrjo G mm. long. The 

 eections pass through the region of the first and second visceral clefts. Tiie 

 distance of the nearly parallel lines indicates the relative thickness of the ecto- 

 derm. 



Fig. 12. The outlines of a series of sections througli the region of the cleft 

 of an embryo 8 mm. long. (Since the complete series is quite numerous, only 

 a certain number have been selected for reproduction. Their positions in the 

 series are indicated by the numbers accompanying each outline.^ The sections 

 are cut somewhat obliquely to the plane of the cleft : they incline from above 

 downwards and backwards. The successive sections from the left to the right 

 correspond to successive planes from before backwards.) 



Fig. 13. A cross section of an embryo 8 mm. long, which shows the bridge of 

 epithelial cells that separates the inner from the outer portion of the cleft. 



Fig. 14. A series of cross sections through the region of the first cleft of a 

 12 mm. erabrj'o. 



Fig. 15. Frontal section of tlie first cleft of a 12 mm. stage, showing the 

 mesoderm which increases the thickness of the bridge. 



Fig. IG. A series of sections from an embryo IG mm. long, cut parallel to the 

 cleft. 



Fig. 17. A series of sections, also parallel to the cleft, from an 18 nun. embryo. 

 They show a deepening of the ventral depression, and a ciiange in the ilirection 

 of the plane of the bridge which separates the inner from the outer portion of the 

 cleft. 



Fig. 18. Transverse section through the region of the car from an embryo 

 2f) mm. long. Tlic bridge has become thinner, and its long axis is now nearly 

 perpendicular to the surface of the head. 



Fig. 19. A jjortion of the section shown in Fig. 20, more highly magnified. 



Fig. 20. Section from a more advanced embryo (30 mm.), made in the same 

 direction as in Fig. 18, but a little more posterior. 



Camdridge, June, 1883. 



