FOURTH PERIOD OF DEVELOPMENT. 151 



in pairs and similar, is already to be quite clearly 

 recognised. At the end of this period of development 

 the difference of formation is so considerable that 

 without knowledge of the development the original 

 similarity of both formations could hardly be imagined 

 (Figs. 60, 61). 



The diverticulum on the left side lies obliquely 

 under the iiotochord, so that its blind end reaches 

 over to the right side. This end, even before the 

 perforation of the outer opening, begins to differentiate 

 into two parts. These are a larger, wider, and more 

 strongly ciliated section, which lies towards the left, 

 and receives the outer opening later on, and a smaller 

 and narrower one having a direction towards the 

 right, which also forms the blind end of the organ. 



The perforation of the outer opening of the inter- 

 nally ciliated organ falls under the post-embryonal 

 period of development, following, as it does, shortly 

 after perforation of the mouth opening. 



The results arrived at through examination of the 

 living object are confirmed and completed by the 

 study of osmic-carmin-glycerine preparations. The 

 study of transverse sections, especially, supplies us 

 with information as to their first alterations, their 



