I9O J. THOMAS PATTERSON. 



more or less retain their individuality, thus producing a number 

 of small vesicles, some of which may, ho\ve\vr, IUM- together 

 (Fig. 8). 



IV. SOME GENERAL CONCLUSIONS. 



We find no evidence in Graffilla that the two embryos commonly 

 found within a capsule are the product of a single fertilixed egg. 

 On the contrary, it is clear that they spring from two ova, which 

 have become enclosed within a common envelope. In tlii- 

 respect our species does not present anything unusual; for while 

 it is the rule among the rhabdocceles to have one embryo in a 

 capsule, yet there are a number of well-known exceptions to 

 this. In his excellent monograph on the turbellaria Von Graff, 

 '08, has recently given a list (p. 2338) of these exceptions, which 

 are as follows: Gyratrix hermaphroditus, Provortex, Collastoma, 

 Umagilla, Polycystis, Fecampia, and Monocells lineata, each has 

 two embryos in a capsule; Anoplodium, 1-2; Prorhynchus stag- 

 nalis, 1-3; P. balticus, 6; Graffilla, 2-3; Promesostoma marmo- 

 ratuw, 4-7; Dalyellia truncata, millportiana and viridis, 4-12; 

 Plagiostomum mttatum and girardi, 10-12; and finally, Syndesmis, 

 2-13. All of this goes to show that the facts which we have 

 brought forward concerning the method of reproduction in 

 G. gemellipara arc entirely in harmony with what is known to 

 occur in the other turbellaria. Even the manner in which the 

 two ova become surrounded by nurse cells within the reproductive 

 glands presents nothing new (unless it be in those cases in which 

 the ova for a while lose their individuality). Furthermore, the 

 habit of directly freeing the ova, with their nurse cells, into the 

 mesenchyme is also seen in such forms as Dalyellia viridis and 

 Olisthanella obtusa. In most forms in which two or more eggs 

 are enclosed within a capsule the ova become surrounded by a 

 common follicle (A nurse cells before they pass to the uterus, 

 where the shell or true capsule is usually secreted. 



Some of the rather rare conditions seen in G. gemellipara are 

 the indefiniteness of the reproductive ducts, the rudimnnarv 

 state of the reccptaculum scminis, the failure of tin- i-gg^ to 

 enter the uterus, and consequently the probable secretion of tin- 

 shell by the mesenchyme. But all of these conditions arc in- 

 cident to the viviparous mode of reproduction. Lin ton 



