Question

Why germinal layers are not present in adult animals?

Answer 1

Germinal layers are roots to the $\textbf{Development}$ of an adult animal.

An Adult animal is the final stage by which a embryo undergoes development and retains higher functioning tissues. Germinal Layers is the end process of Gastrulation in a embyonical life, after it has been done the Germinal layers are no longer going to be present on adults, instead they undergo series of formations to provide the Base for forthcoming systems.

$\boxed{DETAILED \: \: EXPLANATION}$

Gastrulation is the process that helps the direct formation of $\textbf{Germinal Layers}$ or forms $\textbf{Main Germ layers}$ which are three in number, or, three germ layers in total.

The Sum total of all the required processes which are contributing in the conversion of a solid ball of cells or hollow ball of cells or many layered discs of blastula (Multicellular ball-like embryonical structure produced usually at the end of cleavage and is having a fluid filled with blastocoel, called blastula) into two (observed in coelenterates) or in three (starting and observed from platyhelminthes to other groups and/or species) germinal layers of gastrula is summed up as Gastrulation.

This is the most early phase regarded in the most early embryonical development of select advancements of organs in a organism. Gastrulation takes place by the migratory or formative or morphogenetic movements of several blastomeres from the surface of blastula to the properly positioned place in gastrula. All the movements are classified as the following. The movements of gastrulation have seen the involvement of an embryo as a whole, here, the coordination of other movements occurring throughout simultaneously must be in cordial with cellular migrations on some applied parts of an embryo undergoing gastrulation (in short, attributed to different practicalities):

a) Epiboly: Not occurring in embryos of humans, as, Epiboly is never seen. Epiboly is the growth of one part over another part like prospective ectoderminal layers spreading over the rest of the embryos body except the blastomeres.

b) Emboly: These are morphogenetic movements which contribute and promote the migration of ectoderminal layer, mesoderminal layer and primarily notochord cells (important component) from the surfaces to the interior cells. The emboly may (or may not) occur by the way or direction of: Involution, Invagination, ingression and delamination.

Destiny or the Fate of the three Germinal layers

1) Starting layer Ectoderm forms (when fully developed): Central Nervous System, Nerves of the body, retina is formed, lens is formed, cornea of the eyes is formed, conjunctiva formed, ciliary and iridial muscles formed, lining of the nasal chambers are done, membranous labyrinth developed, epidermis formed, cutaneous glands are formed, hair rise from different regions, nails on fingers and legs are formed, claws developed (not humans), hypothalamus developed, pineal gland developed, neurohypophysis or posterior pituitary glands developed, adrenal medulla developed, salivary glands formed and developed, stomodaeum formed for further development and transition into mouth and anterior pituitary gland, and, proctodaeum formed (Ectoderminal or surface ectoderm part of alimentary canal forming a canal or anal canal) and enamel of teeth is also developed from the ectoderm.

2) Mesoderm (or Mesoderminal Layer) forms and/or develops or helping formations of: Derminal (or Dermal layer) of the skin, connective tissues, muscles, skeleton or lays a foundation for skeletal system, notochord formed, blood formed, heart formed (developed afterwards), blood vessels formed, adrenal cortex developed, urino-genital system developed excepting the part of urinary bladder, lining of coelom done, spleen and eyes formed (except the following of ectoderm, lens, cornea and retina), sexual organs or gonads (Reproductive Organ of Male: Testis ; Reproductive Organ of Female: Ovary), Development of Brain (via neurulation and some by first germinal layer called ectoderm).

3) Endoderm: Mesodaeum or mid gut developed, digestive glands developed (exception would be salivary glands as already formed and developed from ectoderm), liver formed, pancreas formed, middle ear developed, eustachian tubes formed (in the middle ear), lining of urinary bladder done, respiratory system developed, adenohypophysis or anterior pituitary gland developed, thymus developed, parathyroid and thyroid glands formed, lining of vaginal layer and urethral layer done, prostate glands developed.