12 EMBRYOLOGICAL MONOGRAPHS. 



7. Older embryo, in which a "horn-like sheath " {9} (pevisare) has formed over the surface of the bud. eb. Epi- 



blast of hydroid. hb. Hypoblast of the hydroid. hb. gm. Hypoblast of bud (?). Tlus may be epiblastic. 

 tb. Tube in profile, tb'. Tube in face. 



8. Older bud, in which hypoblast and epiblast are well marked, e b. Epil)last. h b. Hypoblast. The circular 



canal (can. crc.) is beginning to form by an approximation of two radial chymiferous vessels on each side, 

 a;', x^. Infolded outer wall. 6. Perisarc. 



9. In tliLs bud, .somewhat older than tlie preceding, the circular canal (can. crc.) has formed by a coalescence of 



the exti-emities of the railial tubes, e 6. Epiblast. A 6. Hypoblast, i. «'. Internal bell wall. 7ii. w. Middle 

 bell wall. man. Manubrium, ta. Tentacle. 



10. Older bud, in which the circular canal (can. ere.) is fully formed, and a velum is developed, eb. Epililast. 



ex. ta. External wall of the tentacle, ex. man. External wall of the manubrium, er. vcl. External wall 

 of the velum, i. man. Internal wall of the manubrium (?). i. la. Internal wall of the tentacle, i. vel. 

 Internal wall of the velum. 0. Perisarc. 



11. Older medusa bud, with tentacles coUed up in the future bell cavity, cini. crc. Circular canal, c b. Epiblast. 



ex. man. External wall of the manubrium, ex. ta. External wall of the tentacle, ex. vel. External wall 

 of the velum, i. man. Intern.al wall of the manubrium, i. la. Internal wall of the tentacle, d. Perisarc. 



12. A constriction takes place at the point where the medusa bud rises from the hydroid, and the bud is sepa- 



rated from its attachment and swims away. The perisarc is ruptured to allow this escape. Free meilusa, 

 called Sarsia, which, according to Agassiz, develops from the buds of Corync (tentacles cut oil), fd. Folds 

 in the side walls of the bell. man. Manubrium, or. Mouth, lb. Chymiferous tube. ir. Kemnant of a 

 tube which formerly connected tlie cavity of the hydroid aiul that of the njanubrium. 



13. 0\der Sarsia. man. Manubrium ; the upper letters (man.) indicate a bulbous enlargement of the cavity 



of the manubrium where it joins the hydroid. can. crc. Circular canal, ur. Jlouth. la. Tentacles. 

 ubr. Umbrella, vd. Velum. 



14. Coryne, showing the relationship of the medusa-buds (future Sar.iiir) to the tentacles of the hydroid. mds. 



gm. Medu.sa buds. }>y. Hydro-polyp axis. py. la. Polyp tentacles. 



15. The male hydroid ; the almost perfect medusa is persistent, "developing the spermatic mass around the pro- 



boscis to an enormous extent," Ag.assiz. or. Mouth, py. la. Tentacles of the hydroid. py. s. .Stem of 

 the polyp, man. Manubrium, "loaded with sperm." la. Tentacle. <r. Point of attachment of bud 

 (gonophore ^ ) to the hyilroid. 



16. A more developed male gonophore, which, according to Agassiz, is persistent, and has discharged the sper- 



matic contents, vel. Infolded velum, a. Point of attachment of the umbrella to the hydroid. 



17-23. llyhocodon pirolifcr, Ag. 



17. A profile view of the head of the hydroid, crowded with medusa bu<ls (mih. gm). py. ta. Peripheral row of 



tentacles, py. ta'. Middle row of tentacles, jnj. Hydroid axis. d. Ferule-like enlargement of the perisarc. 

 X. XX. Bases of tentacles (peripheral). 



18. Head of a hydroid, with the circle of peripheral tentacles (ta.) cut off. mils. gm. Medusa buds. py. Hydroid 



axis. 6'. Ferule-like enlargement of the perisarc. 

 18'. Head of the hydroid without medusa buds, showing the position of the two rows of tentacles, jiy. la. and 

 py. la'. Tentacles drawn together. 



19. A young bud in earliest condition. 



Id'. The same ; som<nvhat older, c b. Epiblast. It- b. Hypoblast, tb. Chymiferous tubes. 



20. A well developed medusa, just before rupturing its connection with the hydroid, and showing the tentacles 



on one side. eav. Bell cavity, can. crc. Circular canal, e b. Epiblast. A b. Hypolilast. man. Manu- 

 brium, man'. Base of manubrium, the cavity of which is somewhat enlarged, mds. gm. Jledusa biuls. 

 The second and third formed buds have begun to appear, and are lettered in order of appearance, — 

 2 mds. gm., and 3 mds. gm. ta. Tentacle, ta'. Base of tentacle not yet enlarge<l into a bulb. tb. ta. Chy- 

 miferous tube in the same spheromere as that to wdiich the tentacle hangs, tb^. Tube diametrically oppo- 

 site the tentacular tube. tifi. Chyniiferous tube facing the observer, tb*. Continuation of a chymiferous 

 tube into one of the tentacles. ?. Enlargement of the tube at the junction of lb* and the circular canal 

 (caji, crc.). 

 20*. A (female y) medusa partially developed from the base of the tentacle, car. Internal cavity, eb. Kpiblast. 

 lib. Hypolilast. lb., lb'. Chymiferous tubes in profile and in face. 2 mdi. gm. A secondary medusa, bud- 

 ding from the walls of the first. The medusa buds represented in the figure are taken from a medusa 

 already formed. 



21. View of a medu.sa older than the last, still attached to the hydroid (seen looking at the inner face of the 



tentacle), eav. liell cavity, eb. Epiblast. gjn. A small bud, wliicli will probably later develop into a 

 tentacle, hb. Hypoblast, man. Manubrium, man'. Base of manubrium, mds. gm.-S mds. gnu Medusa 



