BY PROFESSOR W. A. HASWELL. oOl 



the formation of a spherical bod}^ containing a number of nuclei 

 with cell-outlines ill defined, though sometimes recognisable. 

 These spherical embryos are still enclosed within a delicate mem- 

 brane which invests the whole ovary. As they reach a size of 

 0*025 mm. or thereabouts, they become free, and eventually 

 develop into the mature Cercaria condition while floating in the 

 fluid in the interior of the Sporocyst. 



In Ainphistoniuni subclavaium according to Loos (9), in Leuco- 

 chloridiiuii para<ioxum according to Heckert (4), and inDistomum 

 duplicatum according to Reuss (H), the formation of the ova 

 originally takes place at any point in the wall of the Sporocyst — 

 a definite ovary only becoming established at a later stage. 

 Whether this may hold good in the case of the Sporocyst from 

 Mytilus lalus with regard to the primary Sporocyst developed 

 from the Miracidium there are no data from which to decide. I 

 have found no Sporocysts, however small, among the hundreds 

 examined in which the formation of ova w^as not localised in a 

 single ovary; but I may not have seen any except those formed 

 secondarily by fission or endogenous formation, or at all events 

 may not have happened to meet with any early primary Sporocysts. 



The cells of the germinal epithelium give rise, however, not 

 only to embryo Cercarise, but sometimes also, though very rarely 

 so far as m}' specimens are concerned, to a new generation of 

 Sporocysts. These become set free in the interior of the parent 

 Sporocyst. From their germinal epithelium, before they escape 

 to the exterior, embryos of Cercariae msiy already have become 

 formed. The young Sporocysts (fig. 3) on escaping readily 

 multiply by binar}^ fission (fig. 4): a constriction appears dividing 

 the Sporocyst into two equal or slightly unequal parts, and by 

 the deepening of the constriction the two parts become eventually 

 completely severed. 



In young Sporocysts the germinal epithelium is of unifornt 

 thickness throughout ; but at a very earl}^ stage an anterior 

 extremity capable of involution becomes differentiated, after 

 which, the cyst increasing in length, the germinal epithelium soon 

 comes to be attenuated except at the anterior and posterior 



