84 mOHARD EVANS. 



{liji) as thill ectodermal outgrowths situated above tlie developiiip; 

 a|)])endages. The secoud soroite {som?) is divided into median and 

 appendicular portions. The appendicular portion grows downwards 

 into the rudiment of the jaws, but is becoming obliterated through the 

 proliferation of its ventral wall to form the muscles of the jaws. The 

 central yolk is devoid of nuclei. 



rig. 12_$/. — Tiiis figure represents a section from the region of the oral 

 papillse. The section shows the tliird somite divided into median 

 and appendicular portions. On the left side the appendicular portion 

 grows towards the ventral aspect, and is situated on the outer side of 

 the rudiment of the nerve-cord. On the same side, at the tip of the 

 oral papillffi, is represented the rudiment of the slime-gland. 



Fig. 12 /i. — This figure represents a section from the region of the ninth 

 somite, which is divided into median and appendicular portions. The 

 appendicular portion on the right side grows towards the ventral 

 surface, but it does not yet open to the exterior. 



Fig. 12 y. — This figure represents a section from the region of the eleveiitli 

 somite, which is undivided on the left side. 



Fig. 12 k. — ^Tliib figure represents a section from the region of the penul- 

 timate somite, which is provided with an immense cavity. The section 

 shows the inner end of the proctodseum cut across. 



PLATE 8. 



Fig. 13 (X 170). — This figure represents a transverse section, passing 

 througli tiic fourth somite (so;«."') from the posterior end of the embryo 

 shown in Fig. 3. The last two pairs of somites in the embryo in question are 

 still solid. Note especially the germinal nuclei {ge, n.) situated in the 

 splanchnic walls of the somites. The left somite in the figure has been cut 

 through the middle, and shows a group of germinal nuclei which have as yet 

 scarcely assumed the structural and staining characters of the nuclei in 

 question. On tlie right side of the figure the section does not pass through 

 the middle of the somite, but slightly in front of it, and at the dorsal corner it 

 actually cuts through the wall of the somite situated in front. The outline of 

 the endoderm, which possesses irregularly shaped nuclei, is quite sharp and 

 distinct from that of the mesodermal somite. 



Fig. li (X 170). — This figure represents a transverse section, passing 

 through the fourth somite from the posterior end (soni.^-^), of an embryo 

 slightly younger than the one shown in Fig. 4. Tiie last two pairs of somites 

 ill the embryo in question have not yet developed a ccelomic cavity. Note 

 especially the germ nuclei situated in the splanchnic walls of the somites, and 

 also note that, as regards size and structure, they represent all stages 

 transformation, from the ordinary mesodermal nuclei to that of fairly advance 

 germinal nuclei. The somite shown on the right side of the figure exiiibits a 



