198 
J. D. F. GILCHRIST. 
fine basement membrane at the base of the ectoderm cells, 
apparently secreted by these cells (PL 13, fig. 17, b. m.). 
This basement membrane was not, however, usually so dis- 
tinct as in the case figured, and it may be formed by the 
endoderm cells described later. 
With reference to the second fact, there are to be seen in 
the developing ectoderm, after the cellular structure has 
been lost, a number of small bodies about the size of nuclei, 
but readily distinguished from them by their black colour. 
These become larger, and are frequently fused together to 
form elongate black masses. That they are ultimately passed 
out to the exterior was evident from some which were 
observed partly protruding beyond the surface of the develop- 
ing embryo. This accounts for the presence in the living 
state of dark particles floating in the space between the 
embryo and the egg capsule, the rotation of the ciliated 
embryo causing them to move about rapidly, so that their 
presence is readily detected. It also accounts for the charac- 
teristic pigment spots of the embryo, which sometimes 
assumed an elongate shape, and formed a ring round the 
anteriorly situated sense organ. 
Certain areas of the ectoderm seem to retain their distinctly 
cellular structure throughout the changes which take place 
in the ectoderm. The most prominent of these is the part 
which appears as a ventral thickening in the embryo 
(PI. 13, fig. 18, v.th.), and which may, as Harmer suggests, 
represent the disc-like face of the proboscis. Its early 
appearance is noteworthy. The sense organ also consists of 
independent cells, as also the glandular cells of the ectoderm, 
but these are not seen at this early stage, and their cellular 
condition may be of much later origin. 
Origin of Endoderm. 
The origin and mode of formation of the endoderm is, 
as already indicated, one of the outstanding problems of the 
development of Cephalodiscus. 
