DEVELOPMENT OF THE POLYPIDE. 



39 



over into the lumen of the polypide-sac. In other cases, e.g., in 

 Paludicella (Davenport), we find at first a solid ingrowth within 

 which a lumen forms secondarily. The bilaminar sac always remains 

 connected by a longer or shorter strand with that part of the body- 

 wall at which the invagination occurred. This strand (only a portion 

 of which is represented in Fig. 20, A and B), is derived from the 

 neck of the primitive polypide invagination, becomes hollow again 

 at a later stage, and then represents the aperture of the tentacular 

 sheath (Fig. 20, C, ts). 



s 



Fig. 20.— Diagram illustrating the development of the 

 polypide (median sections, after figures by J. Barrois 

 and Nitsche). A, sac-like rudiment of the polypide. 

 The separation of the mid-gut rudiment (d) is marked 

 by a constriction. B, development of the tentacle-buds 

 (t) and the oesophageal invagination (oe). C, older 

 ontogenetic stage in connection with the endocyst (e, e). 

 a, external, I, internal layer of the polypide-rudiment ; 

 d, mid -gut rudiment ; e, e, endocyst ; m, mouth ; n, in- 

 vagination for the formation of the ganglion ; oe, 

 oesophagus ; r, retractor muscle ; t, tentacle ; ts, ten- 

 tacular sheath ; a (in C), anus. 



The outer layer of the polypide-sac (Fig. 20, A, a) is continuous 

 ith the mesodermal layer of the body-wall (endocyst), and yields 

 the mesodermal part of the polypide (the lining of the body-cavity, 

 and the larger groups of muscles, &c). The inner layer (Fig. 20, A, i) 

 is derived from the primitive ectoderm, and yields the ectodermal 

 epithelium of the whole polypide and of the tentacle sheath, the 

 nervous system, and the lining epithelium of the whole of the 

 digestive tract. We should be justified in calling this layer the ecto- 

 derm of the polypide, did not the mid-gut epithelium also originate 

 from it.* 



* [Braem, in a recent work on the development of Plumatclla (Bibl. Zool., 

 H. 23, 1897), states that entodermal tissue can be made out in the embryo. — Ed.] 



