106 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
This same vacuolation is characteristio of the fresh-water forms, as con- 
trasted with forms having a marine habitat. The reverse experiment 
has an opposite result, — a reduction of the vacuolation in the individ- 
ual when gradually transferred to salt water. Strictly marine forms, as 
Amceba erystalligera, also show a marked development of vacuolation 
when they are brought into fresh water. These phenomena seem to 
show an increased activity of the protoplasm in the absorption of water, 
and a corresponding increase in the excretory function is also indicated 
by the much greater activity of the contractile vacuole in fresh-water 
as compared with marine forms. The physiological action of the recur- 
rent cleavage cavity of the Pulmonates is strikmgly suggestive of the 
contractile vacuole of the Protozoa. "The morphological distinction, that 
the one is intracellular while the other is intercellular, militates however 
against the homology of the cleavage cavity and contractile vacuole. 
VI BLASTOPORE AND GASTRULATION. 
I have already called attention in the preceding pages to the changes 
in form characteristic of the stages of cleavage there discussed. ‘The 
same causes produce similar changes in the later stages. We encounter 
rounded embryos with a central cavity (Plate VII. Fig. 47), and also 
much flattened individuals (Plate VII. Fig. 50). The latter are more 
common, and very generally present a more or less quadrangular outline, 
the sides of which are parallel to the first two planes of cleavage, i. e. to 
the antero-posterior and transverse axes of the embryo ; the two or four 
mesoderm cells are symmetrically placed adjacent to the posterior sido. 
In the case of two embryos, not figured, the mesoderm occupies a 
different position with reference to these four sides. Instead of lying 
adjacent to one of the sides, it, lies in one angle, a position suggestive of 
the condition found in the forty-four-cell stage of Figures 39 and 40 
(Plate VL), where the primary mesoblast does not seem to have under- 
gone a shifting into the median plane of the egg. In these cases it 
would seem to be necessary to orient the egg after the manner of Bloch- 
mann (’81) and Rabl (79), with the first two cleavage planes cutting 
the axis of bilateral symmetry at an angle of 45°. 
It is in these flattened quadrangular embryos, containing from one 
hundred to one hundred and twenty cells, six or eight of which are 
mesoderm, that the first traces of gastrulation occur. Previous to this 
epoch the ventral face, i. e. vegetative pole, of the embryo has a much 
