IOO2 DEVELOPMENT OF THE ORGANS OF SPECIAL SENSE. 



a cleft, which permits certain portions of the mesoblast to enter the ocular cavity. 

 This cleft, which extends from the pedicle of the optic vesicle to the border of 

 the invaginated cup (//), is known as the coloboma. It is delimited anteriorly 

 as an unpigmented cleft. At the pedicle of the optic vesicle it continues as a 

 furrow to the base of the cerebral vesicle; and in this furrow lies the central artery 

 of the retina. The margins of the coloboma subsequently unite completely; 

 if, however, in rare cases, this union fails to take place, a strip will be wanting 

 in the retina and in the choroidal pigment. There then results a congenital mal- 

 formation, or arrest of development, or coloboma of the choroid and retina. In 

 birds, the embryonal coloboma-cleft does not close at all, but through it a vascular 

 process of the mesoderm penetrates into the interior of the eye; this is 

 the subsequent pecten. A similar condition occurs in fish, in which the espe- 

 cially large invaginated process, consisting of portions of mesoblast and epiblast, 

 persists as the falciform process. 



Why does the primary, pedunculated optic vesicle become invaginated into 

 itself in the form of an egg-cup? Because a sac, derived from the ectoderm, in 

 the fourth week still pedunculated, becomes lodged in the primary optic vesicle 

 (/, L). From this the crystalline lens is formed, whose epithelial origin (from 

 epiblast) is indicated even in later life by its peculiarities of growth. The capsule 

 of the lens is a cuticular formation of the ectodermal cells. The portion of the 

 ectoderm that covers the optic vesicle in front of the lens subsequently becomes 

 the laminated anterior corneal epithelium. The cornea exists as early as the 

 sixth week. The pigmentary layer of the invaginated optic vesicle passes from 

 the margin of the egg-cup over the ciliary body and over the posterior surface 

 of the subsequently formed iris. It is clear that a persistent coloboma must 

 thus give rise to the formation of an unpigmented strip in the iris, or even a cleft, 

 the coloboma of the iris. The substance of the choroid, the sclera, and the cornea 

 is formed from the mesoblast surrounding the rudimentary eye (m) . The capsule 

 of the lens is at first wholly surrounded by a vascular membrane, the capsulo- 

 pupillary membrane. Subsequently, the lens moves further backward into the 

 ocular cavity, but the anterior portion of the capsulopupillary membrane remains 

 in the anterior portion of the eye, and toward it the margin of the iris grows 

 (seventh week) , so that the pupil is closed by this portion of the vascular capsule 

 (pupillary membrane). The vessels of the iris are continuous with those 

 of the pupillary membrane; those of the posterior capsule of the lens are given 

 off by the hyloid artery, a continuation of the central artery of the retina; the 

 veins empty into those of the iris and the choroid. The vitreous body is first 

 represented as early as the fourth week by a large collection of cells between the 

 lens and the retina. In the seventh month the pupillary membrane disappears. 

 It may, however, persist throughout life as an arrest of development (V). 



The Organ of Smell. On the inferior, lateral border of the fore-brain, the 

 epiblast forms a small pit lined with thickened epithelium, which becomes de- 

 pressed toward the brain, but always remains a pit the olfactory depression. The 

 olfactory nerves arise in the epithelium of the pit, and growing centripetally 

 unite with the olfactory lobe. The nasal cavity appears at first as a blind sac; 

 the choanae develop only as secondary formations. 



The Organ of Hearing. On either side of the after-brain an invaginated pit 

 develops from the epiblast, and becomes depressed toward the brain from without 

 the labyrinthine depression. This subsequently becomes entirely closed off from 

 the ectoderm (as in the case of the lens) , and is known as the vesicle of the labyrinth. 

 It obviously represents the vestibular vesicle, from which, in the second month, 

 the semicircular canals and the cochlea are formed by budding. In the same way, 

 the union of the brain with the labyrinth takes place subsequently through the 

 intermediation of the auditory nerve. The first visceral cleft becomes an ir- 

 regularly shaped, relatively small passage; in the sixth week the auditory bones 

 are present. Externally, the auricle develops in the seventh week; at the bottom 

 of the auditory canal the tympanic membrane is formed; the innermost portion 

 becomes the Eustachian tube. 



The Organ of Taste. The gustatory papillae develop in the last period of 

 uterine life; the taste-buds appear only a few days before birth. 



PARTURITION. 



With the growth of the ovum the uterus becomes more distended 

 and its walls richer in muscle-fibers and in vessels. In the last period, 



