532 ALFRED 0. HADDON. 



which is continually dividing into two by transverse constric- 

 tion. Of these the anterior portion separates from the posterior, 

 and becomes connected with a solid mass of cells, which have 

 proliferated off from the external epithelium. This latter soon 

 acquires a lumen, and we have a pair of single-layered closed 

 sacs occupying a distinct prominence of the stolon, Avhich 

 are beginning to be shut off from the general cavity of the 

 stolon by the neighbouring fusiform mesoderm cells arranging 

 themselves into a diaphragm. Some of the scattered fusiform 

 cells of the stolon become cut off, and so pass into the bud; 

 but at the junction of the primitive closed sac, with the pro- 

 liferating epithelium, there is one single mesoderm cell, wliich 

 by division soon forms a small rounded mass, and is apparently 

 concerned with the formation of the generative organs. The 

 laro^er anterior sac forms the intra-tentacular space, and by the 

 involution of its walls produces the tentacles, and of its floor 

 the oesophagus and the hind gut. A central solid invagination, 

 which shortly becomes a hollow sac, is the rudiment of the 

 nervous system. The posterior smaller sac is prolonged and 

 bent upon itself, and becomes converted into the stomach and 

 intestine, communication taking place between the invagina- 

 tions which form the fore and the hind gut. 



In the embryo Hatschek has discovered that a couple of 

 cells separate themselves from the oral side of the endoderm. 

 These form a single-layered sac, which becomes quite detached 

 from the alimentary tract of the embryo, and is connected with 

 a small ciliated invagination of the lateral epiblast; it also 

 possesses a mesoderm coating. This remarkable structure is 

 regarded by Hatschek, with great probability, as the first bud ; 

 and it will be noticed that it contains the three germinal layers 

 of the embryo. Unfortunately, there is a gap between this 

 stage and the earliest of his true stolon buds ; but it seems 

 pretty evident that the primitive single-layered tubular mass 

 of cells mentioned above is the persistent structure derived 

 from the stomach of the embryo. Assuming this to be the 

 case, we have then in every Pedicellina-bud the three 

 embryonic layers, each one of which gives rise to its traditional 



