DEVELOPMENT OP THE TWO MIDDLE GERM-LAYERS. 



107 



example is presented to us in the development of arrow-worms 

 (Sagitta) or Chcetognatha, concerning which observations have been 

 published by KOWALEVSKY, BIJTSCHLI, and the author. 



After the process of cleavage there arises a typical blastula, which 

 after some time is converted into a typical gastrula. While the 

 latter elongates, two folds of the inner germ-layer arise at the bottom 

 of the coelenteron, and grow up parallel to each other (fig. 65). 



Fig. 65. 



Fig. 65. A stage in the development of Sagitta, after KOWA.LEVSKY, from BALKODR'S 



" Comparative Embryology." 

 Optical longitudinal section through a gastrula at the beginning of the formation of the 



body-cavity. 

 7i), Mouth ; al, alimentary cavity ; pv, body-cavity ; bl.p, blastopore. 



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



The ooelenteroii is separated by means of two folds, which protrude from its ventral wall (V), 



into the intestinal canal proper and the two lateral body-cavities (lit), all of which are still 



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

 D, Dorsal side ; f, ventral side ; ak, outer, ilc, inner germ-layer ; mk 1 , parietal, ml?, visceral 



middle layer ; Ih, body-cavity. 



They grow larger and larger, and at the same time stretch over on to 

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

 on the one hand up to the dorsal wall, on the other up to the 

 blastopore, and thereby completely divide the coalenteron into a 

 middle and two lateral spaces (fig. 66 Ih), which for a time communi- 

 cate with each other near the blastopore and along the subsequent 

 dorsum (D) of the embryo. After a short time this communication 

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

 fuse with the adjacent surfaces of the ccelenteron. Of the three 

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

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



