1889.] Segmentation of the Ovum. 765 
cavity, are a few scattered cells, t*he first members of the meso- 
The entodermal sack of the gastrula is known as the archen- 
teron ; other terms are also in use, e.g., mid-gut, ccelenteron, 
Urdarm, etc. The opening is known as the gastrula mouth 
(archistome, Urmund, etc.). The coelenterates preserve the gas- 
trula organization throughout life, but in all higher classes the 
archenteron gives rise, not only to the permanent digestive tract, 
but also to many appendages and derivatives thereof; and more- 
over the gastrula mouth closes over, and in vertebrates the true 
mouth is an entirely new formation which arises without any con- 
nection whatsoever, so far as known, with the gastrula mouth. 
By gastrulation the ectental line becomes the rim of the gastrula 
mouth. 
A line passing through the centre of the mouth and the 
opposite pole of the gastrula is the so-called axis. Now if the 
mouth be elongated, there would at once be a new longitudinal 
axis marked out, and the gastrula would become bilaterally 
symmetrical. If, further, the mouth is pulled out into a slit, and 
in the process of evolution the lips come together and unite in 
their middle part, the animal would .still have the two ends of the 
original mouth left open, and would so acquire two apertures to 
its archenteron, one anterior to ser\^e as mouth, and one posterior 
to serve as anus. This hypothesis of the conversion of a gastrula 
into a bi-laterally symmetrical animal by the elongation of the 
mouth and concrescence of the lips or ectental line, was first 
suggested, so far as I am aware, by Rabl, 36. A very perfect 
exemplification of the process is afforded by the developing 
ova of Peripatus cape7isis as shown by Balfour, j, and Sedgwick, 
•/J, PI. XXXII. Figs. 23-26. There are, however, serious difficulties 
in applying the theory to bilateral invertebrates. I am strongly 
inclined to think that further research will obviate these dififi- 
culties. 
In certain vertebrates and annelids the concrescence of the 
ectental line has been clearly demonstrated, but the process is 
rendered by secondary modifications much more complex than 
that described in the preceding paragraph. 
