288 
SIDNEY F. HARMER. 
four of which occur iu the section ; whilst in fig. 29 the number 
of cells is larger, and a segmentation cavity or blastocoel has 
made its appearance. The embryo at this stage is usually 
much flattened, partly owing to the pressure of larger embryos 
which are found at the same time in the brood pouch. The 
number of embryos in each brood pouch is, however, consider- 
ably smaller than in Pedicellina, and seldom exceeds about 
two or three. In fig. 30 it is seen that a slight depression has 
been formed in the centre of the flattened side, representing 
the commencement of the archenteron. The cells on the side 
of the blastopore are somewhat higher than on the opposite side, 
whilst the blastocoel is almost obliterated, owing to the pres- 
sure of other embryos. In fig. 31 the blastocoel is larger, 
although the invagination has proceeded a stage further. Fig. 
32 and fig. 33 represent sections of other embryos slightly 
older than fig. 31. In fig. 33 the section has just missed the 
blastopore, so that the invagination appears solid, whereas in 
most other cases, a central depression is present from the first 
in the invaginating mass of cells. Fig. 32 shows the appear- 
ance of a large cell in the immediate neighbourhood of the 
blastopore ; this is one of the two pole-cells of the mesoblast. 
The figures 60 and 61 represent two early stages of L. 
Tethyse, drawn with the same magnifying power as those of 
L. Leptoclini. In fig. 60 the blastocoel has just made its 
appearance. In fig. 61 the invagination has progressed to a 
considerable extent, the section passing transversely through 
the archenteron and blastopore, and cutting both of the pole- 
cells, which even at this early stage are completely shut out 
from any share in bounding the archenteron ; they have 
already taken up their definitive position between the epiblast 
and the hypoblast. 
Fig. 34 represents a section of a gastrula ofL. Leptoclini; 
the archenteron is wide, the blastocoel apparently obliterated, 
and one of the two pole-cells is seen in the neighbourhood of 
the blastopore, although the relation of this cell to the two 
germinal layers could not be distinctly made out. 
The fate of the blastopore is difficult to trace with certainty. 
