240 THE EVOLUTION OF MAN. 



EXPLANATION OF PLATES IL AND IIL 



Egg-cleavage and Gastrulation.'' 



ITiese two plates are intended to illustrate, by means of diagrarnmatio 

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

 trulation. Plate II. represents holoblastic eggs (with dotal 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 tlie primi. 

 tive intestine. In all, the letters indicate the same parts : c, parent-cell 

 {Cytula) ; /, cleavage-cells {Segmentella) ; m, mulberry-germ {Morula); h, 

 germ-vesicle {Blastula) ; g, cup-germ (Gastrula) ; s, cleavage- cavi ty ; d, 

 primitive intestinal cavity ; o, primitive mouth; n, nutritive yelk j i, intes- 

 tinal layer (Entoderm) ; e, skin-layer (Exoderiri). 



Fig. 1-G. — Original or primordial egg-cleavage of the lowest Vertebrate 

 (Amphioxus). Fig. 1, parent-cell {Crjtula) ; Fig. 2, cleavage-stage with 

 4 cleavage-cells ; Fig. 3, mulberry -germ {Morula) ; Fig. 4, germ-vesicle 

 (Blashda) ; Fig. 5, the same, in process of inversion {Invaginatio) ; Fig. 6, 

 Bell-gastrula {Archijastrula). 



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 {Aviphigastrula) . 



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 j Fig. 17, Hood- 

 gastrula {Amphigastrula). 



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, gcroi-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 {Morula), and at the same 

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

 Fig. 30, ]Sauplius-gcrm ; the pharynx-cavity has formed in front of the 

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



