EXPLANATION OE PLATE T^—Contimed. 



also the invagination-hypoblast above' the alimentary canal continuous at the 

 sides with the yolk hypoblast. 



Fig. 5, — A transverse section through the head region of an embryo of 

 Stage B. It shows the splitting of the mesoblast and the formation of the 

 medullary plate and notochord. 



Fig, 6. — A transverse section through the trunk region of an embryo at 

 Stage c, showing a slightly more advanced development than the last. 



Fig, 7. — Represents a transverse section through the anterior trunk 

 region late in Stage d. 



EXPLANATION OF PLATE XXL 



List of Refbkences. 



op. Optic vesicle. i^p. Head cavities (numbered in order 1, 2, &c.) 

 vc. Visceral clefts. aa. Aortic arches and auditory vesicles. eh. Ex- 

 ternal branchia. mh. Mid braiu. hh. Hind brain. th. Thyroid 

 body. al. Alimentary canal. e'p. Outer layer of epiblast. ep'. Inner 

 layer of ditto. Zeiss. A, obj. oc. No. 3, except for figs. 9, 16, and 17. 



Fig. 8. — Another transverse section in the middle region. This section 

 is cut obliquely, so that the lateral and vertebral plates of mesoblast do not 

 appear continuous with the mesoblast lining the sides of the embryo ; it gives 

 therefore at first sight a false impression. 



Fig. 9.— Enlarged view of the lateral epiblast of fig. G. Zeiss D, ocul. 

 3. a. One point of cell division. 



Fig. 10. — Horizontal longitudinal section through the head of an embryo 

 of Stage F. The section is slightly oblique, and heuce unsymmetrical. It 

 shows the unsegmented head cavity. 



Fig. 11.— Vertical longitudinal section through the head of an embryo of 

 Stage K, showing the relations of the head cavities, aortic arches, and gill 

 clefts ; it is taken too much at the side to show the thyroid. 



Fig. 12. — Transverse section through head of an embryo of Stage i. 



Fig. 13. — Transverse section of head of embryo very slightly older than 

 the preceding figure. 



Fig. 14.— Section through the same embryo as fig. 12, but considerably 

 further forwards. 



Fig, 15. — Transverse section through the head of an embryo of about 

 Stage M. 



Fig. 16.— External drawing of an embryo of Stage d. s. t. Sinus 

 rhoraboidalis. 



Fig. 17.— External drawing of an embryo of Stage i. o. Oral invo- 

 lution. 



