THE DEVELOPMENT OF LEPIDOSIEEN PAEADOXA. 39 



Pig. L — Vertical section through an egg, showing the segmentation cavity 

 at its full development. V 221. 



Fre. 5. — Vertical section through the groove of invagination just after its 

 6rst appearance, (Stage 10)y 511. 



I'iG. 6. — Sagittal section tiirough a very slightly more advanced egg. a. 

 Spherical blastonieres round floor of segmentation cavity, i.t/. Invagination 

 groove, s.c. Segmentation cavity. Va 371. 



Fig. VI. — Similar section through eg^ of Protopterus. D 192. 

 Fig. 7. — Part of the section drawn in Fig. 6 under a higher {)ower, show- 

 ing the groove to lie within the region of coarsely-yoiked elements. 



Fig. 8.— Sagittal section of an es:s: during a later stage of gastrulation, 

 showing folding up of coarsely-yoiked elements against the roof of the seg- 

 mentation cavity. VI C 222. 



Fig. VIII. — Corresponding section through egg of Protopterus. 



A 411. 



Figs. 9 and 10. — Small portions of two sagittal sections through an egg of 



similar age to the last. The sections show the tip of the archenteron; the 



section drawn in Fig. 9 favouring the idea of "splitting," that shown in 



Fig. 10 negativing it. VI li 111 and VI b 221. 



Fig. 11. — Sagittal section through an egg of Stage 12 to show the pene- 

 tration of the segmentation cavity by a continuous s|)onge-work ])rt'paratory 

 to its obliteration. 3* E 232. 



Fig. 12. — Sagittal section through an egg of Stage 13 in which the seg- 

 mentation cavity has become completely obliterated, xx D 262. 



Fig. XII. — Corresponding section through egg of Protopterus. 

 C 263. 

 Fig. 13. — Portion of a similar section to tliat in Fig. 11, to show the 

 characters of the small cell margin. 3* D 211. 

 Fig. 14. -Stage 12. R 441. 

 Fig. 15.— Stage 14. 7* 551. 

 Fig. 1(3.— Stage 14. XXXVII C 542. 

 Fig. 17.— Stage 16. XXXVII E 531. 

 Fig. 18.— Stage 17. XXXIIl 632. 

 Fig. 20.— Stage 21. XXXIV B 531. 

 Fig. 21.— Stage 21. XXXIV C 432. 



These figures form a series meant to illustrate the gradual differentia- 

 tion of the mesodermal, notochordal, and other rudiments, c.c. Indica- 

 tion of split to form central canal, e. Epiblast. e/iL Enteron. e. r. 

 Elements of enteric roof, here with coarsely-grained yolk. h. Hypoblast. 

 m. Mesoblast. tii.e. Ti)ickened ectoderm of medullary plate. vi.g. 



