238 P. G. HJ5ATHC0TE. 



derm. stom. Stomodseum. sub. ffl. Suboesophageal gauglion. mfmm. gl. 

 Supraoesophageal ganglion, y.l. Yolk hypoblast cell. y. sy. Yolk-spherules. 

 ». e. Ventral ectoderm. »./. Ventral flexure. ?;. ^. Ventral plate. 



Pig. 1.— Section through an ovum while still in the ovary. (Zeiss, c.) nuel. 

 Nucleolus, m. Nucleus, r. Deeply-stained ring of first food-yolk. /. Folli- 

 cular envelope of ovum. 



Pig. 2.— Section of ovarian ovum shortly before laying. (Beck f in.) 

 nuc. Nucleolus, nu. Nucleus, y. s. Yolk-spherules. 



Jig. 3. — Section through ovum on first day, shortly after laying, ck 

 Chorion, y. sp. Yolk-spherules, e. p. Central mass of protoplasm, nu. 

 Nucleus. 



FiQ. 4. — The central mass of protoplasm has divided into two. s. m. Seg- 

 mentation mass. Nu. Nucleus. 



Jig. 5. — Section through an embryo on the third day. bl. Blastoderm. 

 s. m. Segmentation masses. 



Pig. 6. — Section through an embryo on the third day, rather earlier than 

 Pig. 5. A segmentation mass has just appeared at the surface. 



Pig. 7. — Early on the fourth day. ec. Ectoderm, en. Endoderm. 



Pig. 8. — Pifth day, showing first formation of mesodermal keel. ec. Ecto- 

 derm. e». Endoderm. 



Pig. 9 a. — Sixth day, transverse section through keel. ec. Ectoderm. 

 m. k. Mesodermal keel. 



Pig. 9 b. — Section through anterior end of same embryo. 



Pig. 10. — Sixth day, keel spreading out into mesoderm, ec. Ectoderm. 

 en. Endoderm. ni. Somatic mesoderm. m. Pirst beginning of splanchnic 

 mesoderm, rem. k. Hemainder of keel. 



Pig. 11. — Seventh day. en. Endoderm. ee. Ectoderm v.^. Ventra 

 plate, s. m. Somatic mesoderm, sp. Splanchnic mesoderm. 



Pig. 12. — Vertical longitudinal section through embryo of ninth day. st. 

 Stomodseum. seg. 1. P'irst body segment, mes. Mesoderm, v. ec. Ventral 

 ectoderm, pr. Prootodaeum. dors. ec. Dorsal ectoderm. 



Pig. 13, — Longitudinal vertical section on tenth day. st. Stomodseum. 

 pr. Proctodseum. mesent. Mesenteron. mem. ex. Membranous envelope. 

 seg. Segment. 



Pig. 14. — Longitudinal vertical section on eleventh day, taken a little to 

 one side of middle line so as to pass through all the ganglia on one side. 

 suprace. gl. Supraoesophageal ganglion, st. Stomodseum. pr. Proctodeeum. 

 mem. en. Membranous envelope, mes. Mesoderm, mesen. Mesenteron. n. gl. 

 Ganglia of nerve-cord. n. Nerve. 



The above fourteen figures were drawn under a Zeiss's microscope with a 



