Bestand:The evolution of man (1920) plate III.jpg

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); s, cleavage-cavity; d, primitive intestinal cavity (archenteron); o, primitive mouth; n, nutritive yelk; 4, intes- tinal layer (Entoderm) ; e, skin-layer (Exoderm).

Fig. 18-24.— Discoidal egg-cleavage of an Osseous fish (Motella? Cottus?). The greater part of the nutritive yelk (n) is omitted. (Cf. Fig. 42, 43, pp. 217, 219.) Fig. 18, parent-cell (Cytula); Fig. 19, cleavage stage with 2 cells; Fig. 20, cleavage-stage with 32 cells; Fig. 21, mulberry-germ (Morula) ; Fig. 22, germ-vesicle (Blastula) ; Fig. 23, the same, in process of inversion ; Fig. 24, Disc-gastrula (Discogastrula).

Fig. 25-30.—Superficial egg-cleavage of a Crab (Peneus). Fig. 26, parent-cell (Cytula) ; Fig. 26, cleavage-stage with 4 cells; Fig. 27, cleavage stage with 32 cells; Fig. 28, mulberry-germ (Morula), and at the same time the germ-vesicle (Blastula) ; Fig. 29, Bladder-gastrula (Perigastrula); Fig. 30, Nauplius-germ; the pharynx-cavity has formed in front of the