478 



TUNICATA. 



In the course of further development a cavity forms in the pharyn- 

 geal rudiment and gradually enlarges (Fig. 246 B, ph) ; this soon 

 opens externally through an aperture (branchial aperture,/) resulting 

 from an ectodermal invagination which forms opposite to the atrial 

 aperture. The muscle-hoops which lie in the neighbourhood of the 

 atrial and branchial apertures now become separated from the muscle- 

 plates. The pharyngeal cavity gives off a pair of flat lamella-like 

 diverticula (I) towards the dorsal side, and these, as Grobben had 

 already observed, embrace the neural rudiment («) laterally. Accord- 

 ing to Uljanin, these diverticula are concerned principally in the 



Fig. 246— Diagrams illustrating three stages in the development of the lateral buds of 

 Doliolwm, seen from the side(i4 and B, after Uljanin ; C, after Grobben). cl, atrium ; 

 d, enteric rudiment; e, atrial aperture: ec, basal ectodermal thickening; es, endo- 

 style-rudiment; g, genital rudiment ; i, branchial aperture ; k, gill ; I, lateral out- 

 growths of the pharyngeal cavity ; m, outline of the muscle-plates ; n, neural rudi- 

 ment ; />, pericardial rudiment; /)/'. pharyngeal rudiment. 



formation of the branchial lamellae. A process (d) running back 

 from the pharyngeal cavity develops into the intestine of the bud, its 

 blind end becoming applied to the wall of the dilating atrial cavity 

 (cl). The latter extends specially towards the sides of the body, so 

 that, as Grobben observed, at a certain stage it resembles a pair of 

 spectacles. Its lateral extensions become applied to the wing-like 

 processes of the pharyngeal cavity (/). This juxtaposition of the 

 atrial and pharyngeal walls gives rise to the branchial lamellae (Fig. 

 24G C, /), in which the gill-clefts then break through. The rudiment 

 of the central nervous system undergoes a transformation closely 



