126 GEOEGE W. FIELD. 



s. m. c. Sperm mother-cell. sp. Spermatozoa, st. Stomodscum. s. v. Scliizo- 

 coel vesicle, v. Ventral, v. d. a. Ventro-dorsal area. w. p. Water-pore. 

 w. t. Water- tube. 



All the figures are camera drawings except Fig. 18, which is a diagram of 

 a reconstruction from serial transverse sections ; and Figs. 25 and 26, which 

 are not drawn to scale. 



PLATE XIII. 



Fig. 1.— Cross-section of an alveolus of the ovary ; the eggs are nearly 

 ready to be discharged. Only one half of the section is figured. Pereuyi ; 

 Mayer's cochineal preparation. X \^5. 



Fig. 2. — Cross-section ot an alveolus of the ovary after discbarge of the 

 eggs. Perenyi; alcoholic borax-carmine preparation, x 145. 



Fig. 3. — Cross-section of an alveolus of the testis. Only a small segment 

 is drawn. The oval cells near the centre (*^.), directly without farther division, 

 become changed into the shape characteristic of the spermatozoa. Perenyi ; 

 Kleiueuberg's hsematoxyliu preparation, x 1000. 



Fig. 4. — Egg in eight-celled stage, showing the bilaterally symmetrical 

 division into a right and a left half, x 400. 



Fig. 5. — An egg in sixteeu-celled stage, showing the relative size of ecto- 

 derm and entoderm cells at this time. Perenyi ; Kleinenberg's hsematoxylin 

 preparation, x 400, 



Fig. 6. — Optical section of living blastula soon after its escape from the 

 egg-membrane, showing first appearance of mesenchyme-cells. X 240. 



Fig. 7. — Same at beginning of invagination. 



Fig. 8. — Mesenchyme formation during the progressing invagination. 

 Optical section of living animal, x 240. 



Fig. 9. — Longitudinal section of completed gastrula. Perenyi; Kleinenberg's 

 hsematoxylin preparation. X 240. 



Fig. 10.— Optical section of the living gastrula, looking down somewhat 

 obliquely upon the blind end of the archenteron, to show the formation and 

 relation of the enteroccels. x 400. 



Fig. 11. — Living specimen, seen from the right side, to show the mode of 

 formation of digestive tract and of the circumoral ciliated band, x 70. 



Fig. 12. — Young larva, seen in ventral view, showing the original relation 

 of the adoral («. o. b.) and the circumoral (c. o. b.) ciliated bands. Picric 

 salt ; Delafeld's ha^matoxylin preparation, x 110. 



