214 ARTHUR E. SHFPLEY. 



EXPLANATION OF PLATES XVIII, XIX, XX, aad 



XXI, 



Illustrating Mr. Arthur E. Shipley^s Paper on " Some Points 



in the Development of Petromyzon fluviatilis." 



Reference Letters. 



a. Anus. a. c. Anterior cardinal, ao. Aorta, au. Ear. aur. Auricle. 

 b. c. Body cavity, bl. c. Blood-corpuscles, bp. Blastopore. br>-br? First 

 to eighth gill-clefts, ir. i. Skeleton of branchial bars. Ar.t;. Vessels of bran- 

 chial bars. c. Cerebral hemispheres, c. g. Ciliated groove, d.f. Dorsal fin. 

 d. I. Dorsal lamella, d. m. Dorsal mesentery, e. Eye. e. g. Egg membrane. 

 ep. Epiblast. /. b. Fore-brain, f. g. Fore-gut. g. Groove between mid- 

 and hind-brain, g. h. I. Left ganglion habenulse. g. h. r. Right ganglion ha- 

 benulae. gl. Glomerulus, g.n. Ganglion cells at base of olfactory invagination. 

 h. Heart, hd. Head. hd. c. Head-cavities, h. b. Hind-brain, hi/. Hypo- 

 blast, i. Iter a tertio ad quartum venticulum. inf. Infundibulum. li. i. 

 Liver tubules. 1. 1. Lower lip. /. f. Lamina terminalis. m. Mesenteron. 

 tn. b. Mid-brain, m. br. Muscle of branchial bar. mes. Unsegmented mesoblast. 

 «««. «ow. Mesoblastic somites, m./. Muscle-fibre of heart. »?._$'. Mid -gut. m. 

 Muscle-plate, my. Myomere, n. Notochord. na. Olfactory invagination. 

 n. r. Neural ridge, nu. Nucleus of muscle-plate, o. e. Ciliated epithelium 

 lining nasal invagination, op. ch. Optic chiasma. oph. Ophthalmic ganglion. 

 op.th. Optic thalami. ojo. o. Optic vesicle, p.g. Postanal gut. pin. Pineal 

 gland, pit. Pituitary body. pr. Primitive streak, r. I. Recessus labyrinthi. 

 s. c. Segmentation cavity, s. cm. Superior commissure, s. d. Segmental duct. 

 sm.pl. Somatopleure. sp, c. Spinal cord. sp. gl. Spinal ganglion, sp. pi. 

 Splanchnopleure. st. Stomodeeum. s. v. Sinus venosus. t. Tentacles, th. 

 Thyroid gland, tr. Trabeculae. tub. Tubule of pronephros, u. I. Upper lip. 

 V. Velum. V. ao. Ventral aorta. ven. Ventricle. v.f. b. Cavity of fore- 

 brain. ». h. b. Cavity of hind-brain. v. r. Ventral ridge in mouth, v.v. 

 Valves of the heart, y. c. Yolk-cells. V. g. Ganglion of fifth nerve. V.g. e. 

 Epiblastic ingrowth to form ganglion of fifth nerve. FII. g. Ganglion of 

 seventh nerve. X. g. Ganglion of tenth nerve. 



PLATE XVIII. 



Fig. 1. — Spermatozoa of Petromyzon fluviatilis. 



Fig. 2. — Segmenting ovum at the completion of the third or equatorial fur- 

 row, e.g. Egg membrane. 



Fig. 3. — Segmenting ovum, showing the next two vertical furrows which 

 have divided the upper cells and are extending into the lower. 



