Before image: EXPLANATION OF PLATES II AND III. EGG-CLEAVAGE AND GASTRULATION.⁷⁸

These two plates are intended to illustrate, by means of diagrammatig sections, the most important differences in animal egg cleavage and gastrulation. Plate II. represents holoblastic eggs (with total cleavage) ; Plate III. meroblastic eggs (with partial cleavage). The animal halves of the eggs (exoderm) are coloured gray; the vegetative halves (entoderm with - putritive yelk) red. The nutritive yelk is perpendicularly shaded. All the sections are perpendicular meridian sections through the axis of the primi- tive intestine. In all, the letters indicate the same parts: c, parent-eell (Cytula); f, cleavage-cells (Segmentella); m, mulberry-germ (Morula) ; b, germ.-vesicle (Blastula) ; g, cup-germ (Gastrula); 8, cleavage-cavity; d, primitive intestinal cavity; o, primitive mouth; n, nutritive yelk; 4, intes- tinal layer (Entoderm) ; e, skin-layer (Exoderm).

Fig, 1-6.—Original or primordial egg-cleavage of the lowest Vertebrate (Amphioxus). Fig. 1, parent-cell (Cytula); Fig. 2, cleavage-stage with 4 cleavage-cells; Fig. 3, mulberry-germ (Morula); Fig. 4, germ-vesicle (Blastula); Fig. 5, the same, in process of inversion (Invaginatio) ; Fig. 6, Rell-gastrula (Archigastrula).

Fie, 7-11.—Unequal egg-cleavage of an amphibian (Frog). Fig. 7, parent-cell (Cytula); Fig. 8, cleavage-stage with 4 cleavage-cells; Fig. 9, mulberry-germ (Morula) ; Fig. 10, germ-vesicle (Blastula); Fig. 11, Hood. gastrula (Amphigastrula).

Fie. 12-17.—Unequal egg-cleavage of a Mammal (Man). Fig. 12, parent-cell (Cytula); Fig. 13, cleavage-stage with 2 cleavage-cells (e, mother-cell of the exoderm; i, mother-cell of the entoderm; Fig. 14, cleavage stage with 4 cleavage-cells; Fig. 15, beginning of the inver sion of the germ-vesicle ; Fig. 16, further advanced inversion ; Fig. 17, Hood-