B – the beginning of the forth week

1 – amnion wall;

2 – cardiac primordium;

3 – blood islets; 4 – oral membrane;

5 – cloacal membrane;

6 – chorionic plate;

7 – tertiary chorionic villi;

8 – chorionic blood vessels;

9 – allantois; 10 – primitive foregut;

11 – primitive midgut;

12 – primitive hindgut;

13 – yolk sac cavity;

14 – primitive umbilicus;

15 – amniotic cavity;

16 – extraembryonic cavity

(chorion’ cavity)

Fig.7. Changes in extraembryonic membranes and the embryo’ correlation in

The process of development

1 – amniotic vesicle; 1a – amniotic cavity;

2 – the embryo body;

3 – yolk sac;

4 – extraembryonic coelom (chorion cavity)

5 – primary chorionic villi;

6 – secondary chorionic villi

7 – connective stalk;

8 – tertiary chorionic villi;

9 – allantois;

10 – umbilical cord

Fig.8 HUMAN PLACENTA

A – General plan of placenta structure (arrows point blood circulation in one of lacunae);

B, C, D – consecutive stages of chorionic villi’ formation;

E – structure of placentar barrier in the first trimester of pregnancy;

F – structure of cotyledon

1 – amniotic epithelium; 2 – chorionic plate; 3 – stem villus; 4 – fibrinoid; 5 – reducing yolk sac;

6 – umbilical cord; 7 – septum between cotyledons; 8 – lacuna; 9 – spiral artery;

10 – decidua basalis; 11 – myometrium; 12 - symplastotrophoblast; 13 – cytotrophoblast;

14 – basal lamina of trophoblastic epithelium; 15 – hemocapillar of chorionic villus;

16 – fibroblast; 17 – mesoderm of secondary chorionic villus;

18 – blood vessel of tertiary chorionic villus.

Fig.10 Gastrulation and differentiation of the germ layers in human

A – the germ disk (15 days);

B – stage of the primitive streak;

C – migration of cells of mesoderm and notochord (17-18 days);

D – trilaminar embryo;

E – neurulation and differentiation of mesoderm (20-21 days)

1 – the primitive node; 2 – the primitive streak; 3 – extra-embryonic mesoderm;

4 – the amnion wall; 5 – the yolk sac wall; 6 – epiblast; 7 – hypoblast; 8 - cloacal membrane; 9 – prechordal plate; 10 epidermal ectoderm; 11 – neuroectoderm;

12 – intraembryonal mesoderm; 13 – entoderm; 14 – notochord; 15 – neural tube’ primordium; 16 – somite; 17 – intermediate mesoderm (nephrotomes);

18 – parietal mesoderm; 19 – visceral mesoderm

Table 7