240 THE EVOLUTION OF MAN. 



EXPLANATION OF PLATES II. AND III. 



Egg-cleavage and Gastrulation. 7 ' 



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

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

 trulation. 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 

 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) ; m, mulberry-germ (Morula) ; b, 

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

 primitive intestinal cavity ; o, primitive mouth ; n, nutritive yelk ; i, 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, 

 Bell-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; 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- 

 gastrula (Amphigastrula) . 



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

 The greater part of the nutritive yelk (ri) 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. 25, 

 parent-cell {Cytula) ; Fig. 26, cleavage-stage with 4 cells ; Fig. 27, cleavage- 

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

 time the germ -vesicle (Blastula); Fig. 29, P>ladder-gastrula (Perigastrula) ; 

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

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



