ON DEVELOPMENT ETC. OF PlIOllONIS. 527 



relation of the uiipuired nepliridial sac to tlie gut will 1)e Ijest 

 imderötood from tlie median saiiittal section uiven in li*;'. ol . 



I may licre Itc allowed to put in a short histiH'ical l'evicw of 

 the niesol)last-foi'mation in the PJioronu larva. 



KowALEWSKY ('67) attril)uled the origin of tJic mesohlast to 

 the eetoljlast. 



INIetschnikoff ('82), Foettixger ('82), and E. Schuetze 

 ('97) confounded the plasmic cor])Uscles with the true mesoldast, 

 and none of them was aware of the presence of the anterior diver- 

 ticula. 



Caldwell ('85) made many interesting observations on the 

 mesoblast-formation. According to his view, there exists no meso- 

 hlast Ijefore the closure of the l)lasto])ore lips (lateral), but it arises 

 later from three distinct sources, viz., 1) the anterior paired 

 diverticulum (entoblastic), 2) the posterior paired diverticulum 

 (ectoblastic) and 3) " the primitive streak " connecting the above 

 two structures. Further it has been declared by him that the 

 body-cavities of the larva arise in two different regions. As to 

 the preoral body-cavity, he writes as follows : " From the time 

 when two or three me.^oblast eelh arc budded off from the diverti- 

 cula on either side, a cavity is prese?it in each mass thus for/Ned. 

 These cavities are the two halves of the hodij-eaviitj (preoral) 

 {I.e., p. '374). On the other hand with respect to the ])osterior 

 Ijody-cavity, he states that " if is formed independently in a 

 paired mass of cells which y row out to the end of the first formed 

 sacs and remain separated from septum " [I.e., p. o7(>). Thus he 

 regards the preoral b(xly-cavity as ai'ising after the enterocœlic 

 type. Lastly the author puts forward in his recapitulati(jn the 

 opinion that the blastopore gives rise to both the mouth and the 

 anus. 



