Page 77 - Anatomy and Physiology of Farm Animals, 8th Edition

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(A) (B)
VetBooks.ir Uterine lining Amniotic cavity

Inner cell mass

Epiblast
Trophoblast
Hypoblast

(C)
Primitive streak (D)

Primitive streak Ecotoderm

Mesoderm
Arrows indicate
direction of Endoderm Migrating cells
migrating cells

Figure 3-2. Gastrulation. (A) and (B) Around the time of implantation, when the embryo embeds in the
wall of the uterus, the inner cell mass becomes a disk of two distinct layers, epiblast and hypoblast.
(C) Embryo viewed from above, as indicated by the arrow in (B) (dorsal view). Cells of the epiblast begin
to proliferate and migrate toward the longitudinal primitive streak on the dorsal midline. (D) Dorsal view
of embryo (C) in cross‐section through the region of the primitive streak. Migrating cells move to the
interior of the embryo, where they become endoderm and mesoderm.

Table 3-2. The Germ Layer Origin of Tissues

Ectoderm Mesoderm Endoderm
Epidermis, including Muscle (all types) Epithelia of:
cutaneous glands, hair, nails Cartilage Pharynx, including base of
(claws, hoofs), lens tongue, auditory tube, tonsils
Bone
Epithelia of sense organs, Larynx, trachea, lungs
nasal cavity, paranasal sinuses, Blood, bone marrow
oral cavity Endothelium Thyroid, parathyroids, thymus
Dental enamel Mesothelium (lining of serous cavities) Digestive tube and glands
Nervous tissue Epithelium of kidney and ureter Urinary bladder
Adenohypophysis Epithelium of gonads, genital ducts Vagina, vestibule
Chromaffin cells of adrenal Adrenal cortex Urethra and associated glands
gland
Synovium