vol.3.1 Robertson. — Embryonic Fission in Crisia. 133 



maj be due in part to the time at which the union is effected, 

 i.e., if the bud lias already got started toward the formation of 

 a polypide, the momentum of growth may be so great that the 

 development of the egg lias no power to change or hinder it. 

 Whereas if the union takes place early enough, before bud 

 differentiation has begun, the embryo gains the ascendency, and 

 an ovicell results. 



DEVELOPMENT OF THE PRIMARY EMBRYO. 



The Ovicell. — Development of the embryo in Crisia takes 

 place within a special structure, the ovicell. Smitt ('65) 

 first called attention to the fact that the ovicell of Crisia 

 develops according to the same laws as zooecia, and Harmer 

 has shown that in several genera of the Cyclostomata it is 

 homologous with a zoceciuni. The reasons for these conclusions 

 are first, in Crisia the ovicell occupies a position in the 

 internode similar to that of azooecium. In C. eburnea there are 

 ordinarily seven zooecia in an internode, so that ovicell-bearing 

 internodes consist of six zooecia and an ovicell, the latter taking 

 the place of the second or third zoceeium.* Second, within the 

 ovicell is found a bud which is equivalent to that found in a 

 zoceeium. In the latter this develops into a tentacle sheath and 

 the alimentary canal of a polypide, in the ovicell, into a tentacle 

 sheath and the follicle inclosing the embryo. Third, in Lichen- 

 opora and Tubulipora the ovicell originates in an actual zoceeium. 

 In the former it is the second or third zooecinm of the colony 

 and functions as a brood pouch only after the degeneration of 

 the first polypide; in the latter, any zoceeium may become an 

 ovicell, and after it has already had one or two occupants. 



PI. XIII, Fig. 18, represents in optical section a decalcified 

 tip containing a young ovicell (orl.) in the so-called "funnel 

 stage", in which is a very young embryo {emb.), and the begin- 

 ning of the tentacle sheath {tent.). Starting with the ehitiuous 

 articulation (art.) at the base of the internode, the evicell is 

 fouud in this instance to occupy the place of the third zoceeium. 



*This statement may seem inconsistent with that made on p. 134 relative to the 

 difficulty in locating early ovicell stages, but determinateness in the position of the 

 ovicell is not accompanied by constancy in its occurrence, relatively few internodes 

 possessing ovicells. 



