EXPLANATION OP PLATES III, TV, AND V. 



Fig. 23. — Section through an embryo of Peripatus capensis at the 

 gastrula stage (PL I, fig. 19). The gut-cavity is still traversed by a mass 

 of much vacuolated endoderm. Zeiss's d, oc. 2, camera, bl. Blastopore. 

 ff. g. Gut-cavity of gastrula. ec. Ectoderm, en. Endoderm. 



riG.24,a — d. — Series of sections through a gastrula of Peripatus capen- 

 sis of stage PI. I, fig. 21, before the appearance of the primitive streak. 

 Zeiss's D, oc. 2, camera. 



a. Five sections in front of blastopore, showing increase of nuclei between 

 ectoderm and endoderm, similar to that which takes place at a later 

 stage behind the blastopore. Endoderm cell lying loose in gut. 



b. Through middle of blastopore. Blastopore traversed by strands of 

 protoplasm. 



c. Five sections behind blastopore, showing beginning of polar area iu 

 middle ventral line. 



d. Five sections behind last, through centre of polar area. bl. Blasto- 

 pore, p. a. Polar area. 



Fig. 25, a, h. — Two sections behind blastopore of embryo of Peripatus 

 capensis, slightly younger than Stage A (PI. I, fig. 22). Early primitive 

 streak. No mesoblastic oands. Zeiss's D, oc. 2, camera. 



a. Immediately behind blastopore, bl. Marks position of blastopore in 

 preceding section. 



b. Nine sections behind the preceding. Large increase of nuclei in polar 

 area, constituting the primitive streak, which is marked by a groove. 

 p. g. Primitive groove. 



Fig. 26, a — d. — Series of transverse sections through an embryo of Stage 

 A (PI. I, fig. 22). Mesoblastic bands {m. b.) have begun to grow forward 

 from front end of primitive streak. Zeiss's d, oc. 2, camera. 



a. Through the posterior end of blastopore, and two sections behind the 

 front end of the mesoblastic band. 



b. Hindermost section through the blastopore. 



c. Through the primitive streak three sections behind the blastopore. 



d. Through the primitive streak ten sections behind the blastopore. 

 This section shows a portion of the polar area lying unaltered in the 

 deeper part of the primitive streak, hi. Blastopore, p. g. Primitive 

 groove, p. st. Primitive streak, m. b. Mesoblastic band. 



