BNTOrROCTA. 



93 



During the next stages the development of the definitive form of 

 the body takes place. The cmbrj'o lengthens at right angles to the 

 flattened ventral side (Fig. 44). At the same time the ectoderm 

 thickens over this ventral region, and thus assumes the form of a 

 disc which, at its edge, is sharply marked off from the rest of the 

 ectoderm, and later sinks in to form the vestibule. In the anterior 

 half of this disc an invagination appears which soon becomes lined 

 with ciliated cells (Fig. 44, oe) ; this is the rudiment of the oeso- 

 phagus. At a somewhat later stage another similar invagination 

 develops from the posterior part of the disc (Fig. 44 B, and) ; this 

 is the rudiment of the hind-gut, which at first is a solid, inwardly 

 projecting ectodermal thickening. The oesophageal invagination 



Fig. 44. — Two later stages in the development of Pedicellina (after Hatschek, from Balfour's 



Text-book), ae, arclieuteron ; an.i, anal invagination ; /, ectodermal fold ; f.g, ciliated disc ; 

 oe, oesophagus ; .r, dorsal organ. 



soon becomes connected with the rudiment of the mid-gut (Fig. 

 45 A). The establishment of communication between the mid-gut 

 and the hind-gut, and the acquisition of an external aperture by 

 the latter, occur only in the later stages (Fig. 45 B). 



The mesoderm- cells, meantime, have increased in number, the 

 proliferation of the two pole-cells leading first to the formation of 

 two short mesoderm-bands. Two larval organs specially character- 

 istic of Pedicellina also appear. One of these, which we shall call 

 the ciliated disc (Figs. 44 and 45, fij), is an ectodermal thickening 

 which lies apically and consists of large glandular cells, its margin 

 being beset with stiff cilia. The other larval organ, which is known 

 as the dorsal organ (Figs. 44 and 45, x), lies on the anterior side of 



