240 THE EVOLUTION OP MAN. 



EXPLANATION OF PLATES IX AND m. 



Egg-cleavage and GA8TBULATION. 7 • 



These two plates are intended to illustrate, by means of diagrammatic 

 sections, the most important differences in animal egg-cleavage and gas- 

 trnlation. Plate II. represents holoblastio 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 

 nutritive 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) ; /, cleavage -cells (Segmentella) ; w, mulberry-germ (Morula) ; b, 

 germ-vesicle (Blastula) ; g, cup-germ (Gastrula) ; s, cleavage-cavity; d, 

 primitive intestinal cavity j o, primitive mouth ; n, nutritive yelk ; *, 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) j Fig. 6, 

 Hell-gastrula (Archigastrula). 



Fig. 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) . 



Fig. 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 ; t, 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- 

 gastrula (Amphigastrula). 



Fig. 18-24. — Discoidal egg-cleavage of an Osseous fish (Motella? Cottus 7). 

 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; Jfig. 21, mulberry-germ 

 (Morula) ; Fig. 22, germ-vesicle (Blastula) ; Fig. 23, toe same, in process of 

 inversion; Fig. 24, Disc-gastrula (Discogastrula). 



Fig. 25-30. — Superficial egg-cleavage of a Grab (Peneus). Fig. 25, 

 parent-cell (Cytula) ; Fig. 26, cleavage-stage with 4 oelia ; Fig. 27, oleavage- 

 stage with 32 cells ; Fig. 28, mulberry-germ (Morula), and at the same 

 time the germ-vesicle (Blastula) ; Fig. 29, Bladder-gastrula (Perigastrula) ; 

 Fijf. 30, Nauplius-germ ; the pharynx-cavity has formed in front of the 

 primitive mouth (d), owing to an inversion from without. 



