122 



OHAS. B. WILSON. 



He says (13, p. 480), "1 have not been able to decide upon 

 the origin of the sexual pouches in those forms in which they 

 are preformed (e. g. Drepanophorus, Cerebratulus), but I am 

 convinced that they are of mesodermal origin because they 

 arise as clefts in the parenchyme or the dorso-ventral muscle 

 layer, while there is no possible doubt that the sexual 

 pouches in those forms in which they are not preformed 

 (e. g. Oarinella, Malacobdella), but are developed in conjunc- 

 tion with the sexual products, arise from the parenchyme, 

 and hence are a tissue derived from the mesoderm.^^ 



The development of these pouches in a regenerating papilla 

 bears suggestive testimony toward the same conclusion. 



We have already stated that in regeneration a thin ecto- 

 derm grows backward from the surface of rupture to form 

 the exterior of the papilla. At the same time the entoderm 

 forms a straight tube through the centre, which is of the 

 same size as the central lumen of the intestine since the anus 

 remains at the extreme tip of the papilla, and which is at 

 first without any side pouches {en., fig. 60). The space 

 between ectoderm and entoderm is filled with mesoderm, 

 from which is developed the body musculature. 



The circular muscles and the two longitudinal layers 

 appear first, but are quickly followed by the dorso-ventral 

 muscles. 



A row of the latter muscles, each consisting of several 

 fibres, appears on either side of the intestinal tube and close 

 to it, at about the position they are to occupy when regenera- 

 tion is completed. These form the thickened pillars at the 

 inner ends of the germinal pouches (fig. 60, dvm.). Two 

 lines of smaller vertical muscles extend from these pillars 

 outward to the body-wall, diverging slightly. They are 

 woven together by connective tissue so as to form walls 

 which eventually separate germinal pouches from intestinal 

 C86ca. As the papilla increases in width the walls grow also, 

 each pillar with its diverging walls enclosing a germinal 

 pouch [p.). 



As soon as the pillars and walls begin to form, the ento- 



