MULTIPLICATION BY GEMMATION l6l 



parent, which was developed from an ovum, produces long stalk- 

 like processes or stolons, on which new animals are produced 

 by budding. Each of these stolons is made up of three layers 

 of cells — an outer ectoderm, an inner endoderm, and an inter- 

 mediate layer of motile 'mesoderm cells/ The ectoderm layer 

 gives rise merely to the epidermis of the bud : the epithelium of 

 the alimentary canal and its accessory organs, the branchial sac 

 (• peribranchial tube*), and the pericardial tube, being developed 

 from the endoderm ; and the muscles, ganglion ( ? ), and sexual 

 glands from the ' free mesoderm cells.' 



The endodermal tube mainly determines the form of the 

 animal in these processes : it becomes definitely segmented, 

 and on it the growing ectodermal tube is moulded, so to speak. 

 We may thus conclude that a series of homologous formative 

 zones of structure are to be found in the endodermal tube of 

 the stolon, each of which may consist originally of a single 

 circular layer of cells. At the point where a bud will arise, the 

 corresponding zone of cells growls out to form a bladder-shaped 

 enlargement, which becomes detached from its point of origin 

 on the endodermal tube of the stolon and regularly differenti- 

 ated, so as to give rise to the peribranchial tube, the intestine, 

 and so on. The cells of this endodermic vesicle cannot all be 

 equivalent, nor can they contain exactly similar determinants : 

 were that the case, such a differentiation could not occur, and 

 the walls of the peribranchial chamber could not arise from one 

 part, and the intestine from another. But even as regards the 

 primitive intestinal vesicle itself, one cell must contain the deter- 

 minants of the stomach, another those of the hind-gut, and so 

 on. In short, we must assume that — just as occurs in principle, 

 if not as regards actual details in the case of embryogeny — a dis- 

 integration of the idioplasm and a distribution of the groups of 

 deterniijiants among the different cells takes place during devel- 

 opment. The determinants of all parts arising in connection 

 with each endodermal vesicle, must be collectively contained in 

 each zone of cells of the endodermal tube from which such a 

 vesicle is developed. 



The formation of those organs which arise from the ' free 

 mesoderm cells' of the stolon is the most difficult to understand. 

 There is certainly no reason why we should not suppose that 

 these cells contain very different kinds of idioplasm : one, for 

 instance, might contain ' muscle-determinants,' another • nerve- 



