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AMPHIOXUS. FROG. MAN, FISH. CRAB. 
PLATE I. GASTRULATION. (After Haeckel.) 
Figs. 1 to 17 represent holoblastic eggs (with total cleavage); Figs..18 to 80 show meroblastic eggs (with partial cleavage). The 
animal halves are colored gray, the vegetative halves red. The nutritive yolk is shaded vertically. All the figures show vertical merid- 
ian sections through the axis of the primitive intestine. In all, the letters indicate the same parts: c, the parent-cell (cytula); f, cleav- 
age-cells (segmentelia); m, the mulberry-germ (morula); 6, the germ-vesicle (blastula); g, the cup-germ (gastrula); s, the cleavage-cavity ; 
d, the primitive intestinal cavity; 0, the primitive mouth; n, the nutritive yolk; 2, the intestinal layer; ¢, the skin-layer. 
Figs. 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, bell-gastrula (archigastrula). 
Figs. 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). 
Figs. 12-17 Unequal egg-cleavage of » mammal (man). Fig. 12, parent-cell (cytula); Fig. 18, cleavage-stage with 2 cleavage-cells 
(e, mother-cell of the exoderm; ¢, mother-cell of the entoderm); Fig. 14, cleavage-stage with 4 cleavage-cells; Fig. 15, beginning of the 
inversion of the germ-vesicle; Fig. 16, further advanced inversion; Fig. 17, hood-gastrula (amphigastrula). 
Figs. 18-24.—Discoidal egg-cleavage of an osseous fish (Motella ? Cottus ?). The greater part of the nutritive yolk (m) is omitted. 
(Cf. Figs. 42, 43, pp. 217, 219, Haeckel’s “ Evolution of Man.”) 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 inver- 
sion; Fig. 24, disc-gastrula (discogasirula). ; 
Figs. 25-30.—Superficial egg-cleavage of a crab (peneus), Fig. 25, parent-cell (cytula); Fig. 26, cleavage-stage with 4 cells; Fig. 2%, 
cleavage-stage with 32 cells; Fig. 28, mulberry-germ (morula), and at the same time the germ-vesicle (lastula); Fig. 29, bladder-gas- 
trula (perigastrula) ; Fig. 30, nauplius-germ; the pharynx-cavity has formed in front of the primitive mouth (d), owing to an inversion 
from without. 
