396 I" DONCASTER. 



Explanation of the Lettering of the Figures. 



a. Archenteroii. al. Aliiiieutary canal, al. div. Diverticula of alimentary 

 canal in S. minima. 6. c. 1, 2, 3. Head, trunk, and tail cceloni. bl Blastopore. 

 c. p. Ccelom pouch, c. Eye. ect. Ectoderm, encl.sep. Endodermic septum, 

 epi. Epidermis. /. p., f. t. Posterior and tail fin. /. sh. Ein skeleton. /. r. 

 Fin ray. f. a. Arclientericfold. gang, cb., gang. I., gang. v. Cerebral, lateral, 

 and ventral ganglia, gang. nuc. Nuclei of ganglion, gang. mic. 2. Large 

 nuclei of ganglion, gang. fib. Fibrous layer ("Punktsubstanz "). gen. c. 

 Genital cells. gen. nuc. Nucleus of genital cell. germ. ep. Germinal 

 epithelium, h. Head. hd. Hood. hd. cav. Hood-cavity, h. inus. Head 

 muscles, hhs. Hooks, mes. 1, 2. Mesoderm of head, trunk, nies. so. 

 Somatic mesoderm, mo. Mouth, mus. Muscles, nius. c. Muscle cell, mus, 

 nuc. Nucleus of muscle cell. nuc. g. e. Nucleus of genital envelope, o. 

 Ovum, o.d.c. Lining cells of oviduct, ol. Olfactory organs, ov. Ovary. 

 ph. Pharynx, p. t. Parenchymatous tissue, scp. I. Longitudinal septum. 

 sep. tr. Transverse septum, sep. sjj. spurious septum (in S. minima), sh. 

 E"-"'-shell. t. 0. Tactile organ, v. d. Vas deferens, v. s. Seminal vesicle. 



PLATE 19. 



Fig. 1. — S. bi punctata. Gastrula with genital cells, x 220. 

 Fig. 2. — S. bipunctata. Formation of archenteric folds, x 220. 

 j^je 3_ — s. bipunctata. Embryo with head-cavities recently formed. 

 X 250. 

 Fig. 4.— S. en flat a. Formation of head-cavities. 



Fig. 5. — Sagitta, sp. Optical transverse section of embryo through 

 recently formed head cavities. X 250. 



Fig. G.— S. bipunctata. Embryo shortly before hatching, x 220. 

 YiG. 7. — S. bipunctata. Longitudinal section of embryo during formation 

 of the archenteric folds. X 375. 



Fig. 8. — S. bipunctata. Embryo at stage of Fig. 3. Transverse section 

 through the middle of the body. X 500. 



Figs. 9 — 12. — Four transverse sections through an embryo at about the 

 stat'c of Fig. 3. Fixed with osmic acid. X 600. 

 Fig. 9. — Through the head. 

 I'ig. 10. — Through the ventral ganglion. 

 Fig. 11. — Through the genital cells. 



Fig. 12. — Through the tail region. The archenteric lolda have not 

 yet grown back to this level. 



