Downward, there is an evagination
of the diencephalon, the infundibulum, which gives rise to the stalk and the pars nervosa of the hypophysis.
The cellular mechanisms are well described and are roughly divided into the following phases: (a) Establishment of the shell-founder cell; (b) Invagination then evagination
of the shell field; (c) Successive expansion of the shell field to cover the entire embryo at the veliger stage; and (d) Secretion of the shell matrices and calcification of the shell field .
The pineal gland is localised in a third ventricle evagination
called the pineal recess, and it maintains direct contact with the CSF.
"In vivo" budding of the third frustule was observed by evagination
of the ectoderm layer, whereby the entire process from the onset of budding until the release of the frustule lasted less than 12 hours.
The sciatic nerve involvement has been associated with the "pocket sign" related to a small peritoneal evagination
often situated on the posterior part of the pelvis drawn downward the greater sciatic notch (31).
The sphenoid sinus is recognisable at around the third intrauterine month as an evagination
from the sphenoethmoidal recess and can be seen as a small cavity at the time of birth.
One theory supposes the persistence of a duodenal evagination
involved in the normal development of pancreas, so the remnant part might migrate with the developing of the gastrointestinal tract accounting for its various locations, while another theory suggests the existence of a pancreatic metaplasia of the endodermal tissue .
In addition, the processus vaginalis, an evagination
of the parietal peritoneum, elongates through the internal inguinal ring between the internal and external oblique muscles, creating a path for the descending testis to reach the scrotum.
However, many microorganisms can survive in female insects (absent in adult males) and are incorporated in the later evagination
structures, such as the bulb head, and in the end region of the ovipositor, as occurs in Anobiidae and Chrysomelidae.
The innermost layer, the tunica vaginalis, represents the evagination
of the peritoneum (Tanagho & McAninch, 2000).
It is an evagination
from the walls of the urogenital sinus and is the most common developmental aberration of the female tubular organ with a variable degree of hymenal persistence (Roberts, 1971 and Arthur et al., 1989).
The gallbladder is a secondary derivative of the embryonic foregut, arising as an evagination
of the primitive bile duct that connects the embryonic liver to the developing intestine.