106 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



This same vacuolation is characteristic 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 crystalligera, 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 strikingly 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 side. 



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 VI.), 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 



