36 GEMMATION. 



fleshy lining, continuous with the endocyst ; this internal tunic has numerous large, round, 

 nucleated cells distributed through its substance, and internally it presents an uneven surface. 

 The two tunics of the gemma are to become the ectocyst and endocyst of the future cell. 



By this time the gemma has become considerably elongated and has acquired a clavate 

 form, and its cavity begins to be cut off from that of the parent-cell by the formation of a 

 septum. We next perceive that a portion of the lining tunic, near the wide extremity of the 

 gemma, projects in the form of a roundish mass into the interior of the young cell (fig. 4). It 

 is the rudimental polypide, and we soon find in it a cavity, which is to become that of the 

 tentacular sheath, within which, when viewed in front, may be seen an oval ring (fig. 5) which 

 is afterwards to become the lophophore of the polypide. This ring is at first quite simple, 

 resembling a mere fold of thickish membrane, but in a short time it presents all round 

 a series of minute tubercles (fig. 6), the rudiments of the future tentacula. When viewed 

 laterally (fig. 7), the lophophore in this stage is seen to be decidedly bilateral, being 

 prolonged towards the side where the rectum is to make its appearance. The central space 

 between the rudimental tentacula is prolonged downwards, constituting the first trace of a 

 pharynx ; and immediately below this, the mass of the polypide is hollowed out into an 

 internal cavity, which is to become stomach and intestine. This cavity is at first filled with 

 clear, round bodies, having a high refracting power, but in which I could not trace the double 

 outline of a true cell-wall. The polypide is now seen to be suspended from the wall of the 

 cell by a membranous sac This sac is closely adherent to the polypide behind the 

 lo})liophore, but upon the lophophore and tentacula it has become free, and here constitutes 

 the tentacular sheath. Some delicate fibres, the rudiments of the retractor of the polypide, 

 may now be plainly seen extending from just behind the lophophore to the walls of the 

 cell. The body of the polypide at the spot directly opposite to the rudimental crown of 

 tentacula, is also seen to be connected by a short, thick, fleshy mass, with the walls of the 

 cell. Circular fibres may have been already seen in the lining membrane of the cell ; these 

 are chiefly collected near its proximal end (fig. 6), and are to become the parietal muscles 

 of the adult. The body of the polypide continues to elongate itself (fig. 8), and we 

 can soon distinguish in its cavity the three regions of oesophagus, stomach, and intestine. 

 The rudimentary parieto-vaginal muscles may also be seen extending from the tentacular 

 sheath to the walls of the cell ; the thick, fleshy mass, by which the body of the polypide 

 was connected with the cell-walls, has become lengthened and divided into two chords, the 

 anterior and posterior funiculus ; the tentacula continue to increase in length ; the lophophore 

 loses its bilaterality and becomes orbicular, and we now (flg. 9) find little wanting to give 

 to the polypide and its cell the form presented by the adult. 



Up to this period the young polypide has been entirely shut off from all communication 

 with the external water, and its nutrition must have been effected through the general 

 nutrition of the colony ; now, however, an opening occurs in the new cell just over the tenta- 

 cular crown of the polypide, and the last stage of development is entered on. The tentacular 

 crown rapidly acquires its complete form, the inferior extremity of the alimentary canal 

 becomes elongated into the great cul-de-sac of the stomach, the muscles are by this time all 

 formed, and the polypide is capable of exsertion and retraction. It is now no longer dependent 

 for its growth on the general nutrition of the colony, but has become an independent zooid, 

 obtaining its food from without, and submitting it to the action of its own digestive system. 



