18 BULLETIN OF THE 



of the origins of the muscles away from the neck, to siippose that the 

 circumcervical region is derived in that way; for (1) the origins of the 

 pyramidal muscles actively migrate away from the neck to a certain 

 extent, and (2) the normal growth of the body wall is sufficient to ac- 

 count for the carrying backward of the origin of the retractor's. 



From the facts already gained it seems clear that the ectocyst (cuticula) 

 is first formed at the tip, and then, to meet the wants of the growing 

 colony, this is replaced later by a cuticula of different chemical compo- 

 sition, which becomes thicker as the body wall grows older. At a late 

 stage we find a separation of the thick cuticula itself into two layers, of 

 which the outer one is much the more highly refractive.-' (Plate II. 

 Fig. 13 ; Plate III. Figs. 26, 29.) 



6. Development of the Polypidb. 



We have already (pages 8, 9, Figs. 5, 14, 37) seen how the foundations 

 of the polypide are laid by the ingression of cells of the outer layer of 

 the body wall pushing before them the mesoderm, and how, finally, those 

 cells arrange themselves in a boat-shaped mass to form the inner layer 

 of the bud (Plate III. Fig. 21), which possesses no actual cavity, and 

 is constantly separated from the external world by the ectoderm which 

 remains behind to form the neck of the polypide. Even when a cavity 

 is formed later, it does not communicate with the exterior until the 

 permanent atrial opening has arisen. The earliest differentiation in the 

 bud is, as mentioned by Allmann ('56, p. 36), the formation of a cavity 

 which is to become that of the atrium. This cavity is first formed 

 at an early stage as an extremely slight fissure in the midst of the inner 

 layer. Figure 22 shows a longitudinal section of this stage. Cell di- 

 vision is taking place throughout the whole mass, but especially at the 

 neck of the polypide, cev. pyd. The position of the cavity is represented 

 by the central non-nucleated space, and this gives rise, as the later his- 

 tory of development shows, to the atrium and the pharynx. , 



Figure 23 represents a stage which is doubtless of short duration, for 

 I have found it only twice. The bud is much more developed at the 



1 Such a two-layered condition of the cnticula was long ago described by Eeichert 

 ('70, pp. 265, 266) for Zoobotryon. He distinguished " eine aussere, fesfere, starker 

 lichtbrechende und spriidere Schicht und die innpre weicherc." Realizing that the 

 "ectocj'st" or cuticula undergoes many changes in form, — formation of lateral 

 buds, of septae or communication plates, and increase in size of the stolon, — he 

 suggested, without having observed the process, that probably during these 

 changes the more rigid outer layer disappeared and was replaced by the inner 

 softer one. 



