STRUCTURE AND DEVELOPMENT OF LOXOSOMA. 335 
Fig. 27. — Drawn living, as seen through the walls of the vestibule of the 
adult, vm. Vitelline membrane. 
Fig. 28. — A section of an embryo consisting of about eight cells. 
Fig. 29. — Blastosphere stage, blc. Blastocoel. 
Fig. 30. — Commencement of the invagination ; the blastocoel is almost 
obliterated. 
Fig. 31. — The invagination has progressed to a considerable extent, but 
the blastocoel is much larger than in Fig. 30. 
Fig. 32. — A slightly older embryo, pm. Pole-cell of the mesoblast. 
Fig. 33. — A similar stage, the section having missed the blastopore and the 
pole-cells. 
Fig. 34. — The invagination is here far advanced, the blastocoel obliterated. 
The blastopore appears to be filled up by one of the pole-cells, which really, 
however, lies in a somewhat deeper plane of the section. 
Fig. 35. — A nearly median longitudinal section of a stage after the forma- 
tion of the stomodeeum, whose position in a neighbouring section is indicated 
by the line m. The ventral gut flexure is already acquired ; at the posterior 
end are two mesoblast cells. 
Fig. 36. — Another section of the same embryo, v. is the commencement 
of one of the vestibular invaginations ; og. is the oral groove. The cells 
which fill up part of the blastocoel belong in part to one of the lateral walls of 
the stomach. 
Fig. 37. — A more advanced embryo in median longitudinal section, z. 
Junction of stomodseum and meseuteron. s. The sucker, br. The epiblastic 
thickening forming the dorsal organ or brain. 
Fig. 38. — A stage slightly further advanced in development. The brain 
(br.) consists of two layers of cells (neither this section nor Fig. 37 has passed 
through the intestine). 
Fig. 39. — Obliquely longitudinal section of an older embryo, cutting the 
oesophagus ( ce .), stomach (st.), sucker («.), brain (br.), and one of the vesti- 
bular invaginations (v.). 
Fig. 40. — More advanced than Fig. 39. The section passes at some dis- 
tance from the middle line, but parallel to the symmetrical plane of the 
embryo, og. Oral groove, v. Vestibular invagination, br. One of the 
lateral wings of the brain (compare with fig. 46). st. Wall of the stomach. 
mes. Mesoblastic band. The blastocoel contains in addition one or two meso- 
blast cells which in the section do not appear connected with the mesoblastic 
band. 
Fig. 41. — A somewhat similar section, passing obliquely in such a manner 
as to cut only the more ventral regions of the embryo, and thereby to avoid 
the stomach. The blastocoel contains numerous mesoblast cells, mus. is a 
muscle-cell, branched at one end. 
Fig. 42. — Another section of the same embryo avoiding the brain, vestibule, 
