424 LEFEVICE. 
EXPLANATION OF PLATE XXXII. 
Fic. 25, a, 6, c,d, and e. Sections illustrating the development of the pericar- 
dium. a is from a frontal section at a very young stage, and shows.the rudiment 
when it consists merely of a few cells loosely attached to the inner vesicle. The 
remaining figures are drawn from transverse sections. In 4 and c the rudiment 
has increased in size; cells are being added from without, and nuclei within the 
mass are seen to be dividing. In d@ and ea cavity, c.fc.r., has appeared in the 
center of the rudiment, which is now much larger, and the cells are becoming 
arranged into a one-layered epithelium. X 700. 
Fic. 26, a, 4,c,d, and e. Transverse sections representing stages in the develop- 
ment of the dorsal tube. In aa few cells are seen loosely grouped together, and 
lying on the wall of the inner vesicle. In 4 the association with surrounding blood 
cells is apparent. Inc and d@the mass has become compact, and cell-boundaries 
have entirely disappeared ; in the latter figure the lumen in the center has begun 
to form, /.d.¢. In e the tube is completely formed, and the cells are arranged 
around the lumen in a one-layered epithelium. X 700. 
Fic. 27, a, 6,¢, d,ande. Transverse section showing the development of the 
ganglion. Ina and @ a few cells resembling amoeboid cells of the blood are seen 
lying on the dorsal side of the dorsal tube, d.¢. The outer membrane of the latter 
is broken at this point, and nuclei appear to be wandering out into the rudiment, 
gir. Inc and d the rudiment is much enlarged, and in the latter the nuclei are 
beginning to arrange themselves peripherally. In e¢ the wall of the dorsal tube 
under the ganglion has been re-formed, and the central portion of the latter is now 
free from nuclei and occupied by fine fibrils. X 700. 
Fic. 28, @ and 6. Stages in the formation of the branchial orifice. Ina the 
much thickened invaginated ectoderm, ¢.47.0., is seen nearly touching the branchial 
wall. The fusion of the two walls is shown in 6. X 700. 
_ Fic. 29,@and 4. These sections illustrate the development of the branchial 
stigmata. In a@the thickened branchial wall, 47.w., is slightly evaginated, and is 
in contact with the thickened patch of cells in the visceral wall of the peribranchial 
sac, £6.w. The lower portion of 4 shows the fusion between the two walls, and 
the upper portion, a stage after the formation of the opening. X 7o0. 
Fic. 30. Section through the wall of the stomach, s¢.w., and a portion of the 
duct of the “‘organe réfringent,” o.7.7., showing clearly the connection of the latter 
with the digestive tract. X 700. 
Fic. 31. Transverse section of the terminal portion of the intestine, zvz., 
showing the surrounding ducts and ampullae of the “ organe réefringent,” with their 
deeply stained nuclei, amg. X 700. 
Fic. 32, a, 6,andc. Sections illustrating the early development of the sexual 
organs. In q@an early stage is shown when the rudiment, ¢.7., is merely a loose 
mass of cells surrounding a slight cavity, c.g. A couple of the elongated cells 
whieh will help to form the genital cord are seen at g.c. In d the cavity of the 
rudiment is much enlarged, and the genital cord is prominent, and attached to the 
sphere; in both these figures the genital rudiment is seen to lie close to the wall 
of the digestive tract. Inc an older stage is shown, in which the cavity,of the 
sphere has become divided into two, c and ¢’; the genital cord, although only 
represented in part, now extends nearly to the posterior wall of the cloaca, and 
lies close to and parallel with the intestine. X 700. 
