-
Fertilization
When semen is deposited in the vagina, the spermatozoa travel through the cervix and body of the uterus and into the Fallopian tubes. Fertilization of the ovum (egg cell) usually takes place in the Fallopian tube. Many sperm must cooperate to penetrate the thick protective shell-like barrier that surrounds the ovum. The first sperm that penetrates fully into the egg donates its genetic material (DNA). The egg then polarizes, repelling any additional sperm. The resulting combination is called a z -
Morulae
The term morula is not historically appropriate for mammals, however, because the amphibian morula gives rise to embryonic tissues only, whereas in mammals non-embryonic structures (such as the chorion and the amnion) are also derived from the initial mass of cells. -
Blastocyst
The blastocyst is the hollow mass of cells from the initial appearance of the cavity (stage 3) to immediately before the completion of implantation at a subsequent stage. The blastocystic cavity, under the light microscope, begins by the coalescence of intercellular spaces when the organism has acquired about 32 cells. In in vitro studies, a cavity formed in some human embryos at 16-20 cells (Edwards, 1972). -
Criteria
Although the criteria for the first three stages are those of the first three horizons, it has not proved practicable in stages 4-10 to retain the criteria for horizons IV-X. This abandonment had already been begun by Hertig, Rock, and Adams (1956) and by Heuser and Corner (1957). -
Embryos
comprises embryos that are implanted to a varying degree but are previllous, i.e., that do not yet show definite chorionic villi. Such embryos are believed to be 7-12 days old. The chorion varies from about 0.3 to 1 mm, and the embryonic disc measures approximately 0.1-0.2 mm in diameter. The significant dimensions of Carnegie specimens of stage 5 are listed in Table 5-1. The external and internal diameters of the chorion are listed as “chorion” and “chorionic cavity,” respectively. -
Chorion
Axial features are not evident, or at least have not been described, in all embryos of stage 6. Moreover, in some instances, their presence is in dispute. It is possible, however, if the fixation and plane of section were always suitable, the series complete and free from distortion, and an adequate search made, that axial features would be found. Thus, in the well-known Peters specimen, which is frequently considered not yet to show axial features, the possible presence of an allantois was rais -
Amnion
The amnion consists of two layers of squamous cells: internally the amniotic ectoderm, and externally an interrupted stratum of
Page 54 cells resembling mesothelium. Between the two layers, some mesenchyme may be seen in places. -
Endoderm
The endoderm presents gentle elevations and depressions as seen from the ventral aspect, and its dorsal surface, “due to numerous extensions which lead toward the mesoderm, has the appearance of a range of mountains” (Heuser, 1932b). -
Mesoderm
The mesoderm, as the mesoblast may now be termed, is arranged on each side as (1) a longitudinal, paraxial band (nicely shown in Ludwig’s reconstruction, plate 2, figs. 3 and 4), and (2) a lateral plate. The paraxial mesoderm is beginning to become segmented in the junctional region with the spinal cord, i.e., RhD (Müller and O’Rahilly, 1983; O’Rahilly and Müller, 1984b). -
Notochordal plate
Although frequently referred to as the notochord, the axial cells caudal to the prechordal plate are still merely notochordal plate at this stage. The notochord sensu stricto is present only where notochordal cells have become completely separated from a continuous endodermal lining (Müller and O'Rahilly, 1985), and this does not occur until the next stage. -
Zona pellucida
In stage 3 the zona pellucida may be either present or absent. In vitro, the blastocyst emerges from the zona at about 6-7 days. The emergence is commonly referred to as "hatching." -
Somites
An apparent rotation of the somites takes place between stages 11 and 12 (O'Rahilly and Müller, 1984b). The longitudinal axis of the dermatomyotome, as seen in cross section, comes to make a more acute angle with the median plane. Associated with this, the dorsal surface of the body, as seen in cross section, is changing from a gentle to a steeper curvature. -
Upper and Lower limb buds.
This is the stage when one can first see both upper and lower limb buds. As was true in stage 12, the central nervous system seems to be the chief factor in determining the form of the embryo, because the external contour of the neural tube is the same as that of the embryo. Otherwise the most prominent surface characteristics of the group are supplied by the series of bulging condensed masses, the pharyngeal arches, and the similarly appearing whitish opaque limb buds. To the rear of the hyoid -
Opaque
The opaque unsegmented band of mesenchyme between the upper and lower limb buds is evident. The mesonephros lies directly ventral to it. The otic vesicle is clearly visible, as is also the Nackengrube. -
Postovulatory
Up to this time the central nervous system appears to have played the principal role in determining the contours of the head and trunk of the embryo. Other details in its form are provided by the heart, the limb buds, and the condensed masses that are to form the mandibular and hyoid regions (the pharyngeal arches). As in stage 14, the embryo resulting from these influences is bilaterally flattened, having a curved or partially spiral axis. In the embryos of stage 15, the relative width of the t -
Cervical sinus
The cervical sinus, an indication of which may be detected in some embryos at stage 13, is clearly visible by stage 14. Its floor is formed by arches 3 and 4 (O’Rahilly and Müller, 1985). The cervical sinus is smaller by stage 16. Contrary to Frazer (1926), it would seem that pharyngeal arch 3 is no longer visible on the surface after stage 16. The operculum frequently shown in drawings is incorrect. -
Auricular Hillocks
The auricular hillocks exhibit their characteristic form at stage 17. Shown in figures 16-5 and 17-3, they consist of six circumscribed superficial condensations, three (Nos. 1–3) on the mandibular arch and three (Nos. 4–6) on the hyoid arch. The latter are more prominent and are destined to form the auricle of the ear. The three on the caudal surface of the mandibular bar are less sharply outlined. The ventralmost of them becomes the tragus, and the dorsal two join in the formation of the crus -
Gauge
To gauge accurately the level of development of an embryo, its internal structure must be taken into account. There are many things, however, to be learned from the external form. One can see, for instance, that the group of embryos shown in figure 18-2 is more advanced than the similar group of the preceding stage, shown in figure 17-2. The embryo of stage 18 is larger and has advanced in the coalescence of its body regions, which have now become more closely integrated in a common cuboidal bul -
Trunk
The trunk has begun to elongate and straighten slightly, with the result that the head no longer forms a right angle with the line of the back of the embryo. The limbs extend nearly directly forward. The toe rays are more prominent, but interdigital notches have not yet appeared in the rim of the foot plate. -
Delicate
A delicate, fringe-like vascular plexus now appears in the superficial tissues of the head (fig. 20-2). In the temporofrontal region a growth center arches over the eye, and in the occipital region a second growth center occurs above the ear. The edge of the plexus is approximately halfway between the eye-ear level and the vertex of the head. -
superficial vascular plexus
The superficial vascular plexus of the head has spread upward to form a line at somewhat more than half the distance from eye-ear level to the vertex.The fingers are longer and extend further beyond the ventral body wall than they did in the previous stage. The distal phalangeal portions appear slightly swollen and show the beginning of tactile pads. The hands are slightly flexed at the wrists and nearly come together over the cardiac eminence. The feet are also approaching each other, and the -
Eyelids
The eyelids, which have been thickening gradually, are now rapidly encroaching upon the eyes. The formation of the auricle has progressed noticeably: the tragus and antitragus especially are assuming a more definite form. The superficial vascular plexus of the head extends upward about three-quarters of the way above the eye-ear level. The hands extend further out in front of the body of the embryo, and the fingers of one hand may overlap those of the other. -
external genitalia
The external genitalia are well developed but do not suffice for the detection of sex. In particular, some males tend to be diagnosed as females (Wilson, 1926b). Only in fetuses of about 50 mm is it safe to make an assessment.
