Development of the germ cell layers
Within the Iluid-Iilled blastocyst, most of the cells come to lie on one side, forming the inner cell mass. A thinner layer of cells surrounding the fluid cavity form the Irophoblnst (Fig.
11.11). The inner cell mass becomes the embryo and. in the early stages, is a flat plate of cells. The trophoblast becomes the extra- embryonic membranes.The cells now begin to form three germ cell layers. which eventually differentiate into parts of Iheembryo and the different membranes:
The inner cell mass divides into:
- An outer layer or ectoderm - forms the skin and nervous system
weeks 35 mm long; ear flap is distinct; eyelids partly cover the eyes; digits can be seen on the paws; external
genitalia are near to final positions; tactile (sinus) hairs are present on the upper IipJormation of internal organs (organogenesis) is complete
6 weeks 60 mm long; prominent scrotal or vulval tissues; digits widely spread; eyelids are fused; hair follicles and
tactile follicles present on the body; claws present; ossification of skeleton at 45 days
7 weeks IOO mm long; body hair and colour markings are developing
8 weeks 150 mm long; hair covering is complete; pads have developed
9 weeks Ready for birth