M:\ BLOPMBKT Of THE 'i \\ ' M 



107 



example i- presented to us in the dmdopment 



(Sagitta) or ChcetognatAO} oincemini: which oi>-M-vations have been 



puhli>hed liy K'i\\ M.I:\SKV, I Ji TN.-HI.I. and l he ant h<>r. 



After the process of cirri vage there arises a typical 1. last u la, which 



after >ome time is converted int.) a tyj.i.-al ;a>trula. While the 



latter elongates t wo folds ot' tin- inner L'erm-layer arise at the bottom 



be cd'lenteron, and grow up parallel to each other (fig. 65). 





ak 



Ih 



nk- 





Fig. 65. 



V 

 Flf, 



Fig. 65. -A sUge in the development of Sajitti, afc^-r KOWALEVSKY, from BALKOCR'* 



l..ir_'itudiii:il siv-tion through a gastrula at the beginning of the formation of the 

 body-' 



ith ; at, alimentary cavity ; pv, body -cavity ; bl.p, blastopore. 



Fig. 66. Optical cross section through a larva of Sagitti. 



|. :u;itr -I liy nuMiis of two folds, which protrude from its ventral wall (F) f 

 into the intestinal canal proj)er and the two lateral body-cavities (i/t), all of which are still 

 in communication with one another on the dorsal side (5). 



.I Mile; r, ventral side; ak, outer, ik, inner germ-layer ; ink 1 , parietal, ink*, \: 

 middle layer ; Ih, body-. 



^row larger and larger, and at the same time stretch over on to 

 the ventral wall of the larva. From here the free edges finally g 

 on the one hand up to the dor>al wall, on the other up to the 

 l.la^topore, and thereby completely divide the calenteron ii. 

 middle and two lateral >paces (tii:. lii', Ih). which for a time communi- 

 cate with each other near the blastopore and alon^ the subsequent 

 do: sum (/)) of the embryo. After a short time this communication 

 is lost ; the blastopore becomes closed, and the edires of the folds 

 fuse with the adjacent surfaces of the coelenteron. Of the three- 

 cavities the middle becomes that of the permanent intestinal tube, the 

 two lateral ones (Ui) become those of the two body-cavity sacs which 



