THE DEVELOPMENT OF PEEIPATUS CAPENSIS. 
459 
The ectoderm now grows round the endoderm cells and 
entirely surrounds them, excepting at one point. At this 
point, which is opposite to that on which the ectoderm cap 
was placed, the endoderm cells may be seen for a short time 
projecting (figs. 17 and 18). 
The embryo has thus acquired a spherical form, and consists 
of a solid gastrula, the small uncovered spot of endoderm con- 
stituting the blastopore. A cavity next appears in the centre 
of the endoderm cells, so as to open to the exterior through 
the blastopore (figs. 19 and 21). 
We have thus arrived at the stage of a typical gastrula formed 
of two layers of cells, which are continuous with one another at 
the blastopore and enclose a central cavity. It may be at 
once stated that the blastopore, which is on the ventral surface 
of the embryo — on the surface opposite to that on which the 
ectoderm cap was placed — persists and gives rise to the mouth 
and anus 1 of the adult, and that the cavity of the gastrula 
becomes the mesenteron. 
The General Development of the Embryo. 
The segmentation, as we have seen, is complete but unequal; 
the large cells giving rise to the endoderm, and the small cells 
to the ectoderm. The gastrula arises by a modified process of 
epibole. The fully-developed gastrula is shown in figs. 19 and 
21. The embryo has already become slightly oval, and the 
blastopore now begins to elongate in the direction of the long 
axis. 
Stage A (fig. 22). — An opacity appears at the hind end of th 
blastopore. This opacity is the primitive streak. It appears 
to be due to the active proliferation of some cells, which cannot 
1 These were called in Balfour’s memoir (this Journal, 1888), and perhaps 
more correctly, the embryonic mouth and anus, — more correctly because 
they come in the adult to lie internally, in consequence of the ingrowth of 
ectoderm at the two ends of the alimentary canal to form the stomodaeum and 
proctodaeum. They constitute in the adult the openings between the mesen- 
teron and the stomodaeum and proctodaeum respectively. It must, however, 
be borne in mind that they never become closed. 
