Language Of Embryology
The language used to and the developmental processes that mould it is necessarily descriptive. It is similar to anatomical terminology, but there are some common differences that the reader should be aware of.
The embryo does not, and for most of its existence cannot, take on the anatomical position. The embryo is more curved and folded than the erect adult. The adult anatomical position is described as the body being erect with the arms at the sides, palms forward and thumbs away from the body (Figure 2.1). The anatomical relationships of structures are described as if tion, so for the embryo we need to rethink this a little.
Anatomically speaking you may intercangeably use cranial or superior, and caudal or inferior. Cranial clearly refers to the head end of the embryo and caudal (from the Latin word cauda, meaning ‘tail’) refers to the tail end (Figure 2.2). If you imagine the early sheet of the embryo with the primitive streak (see Chapter 14) showing us the cranial and caudal ends, you can imagine that it can be clearer to use these terms rather than superior and inferior.
The term ‘rostral’ may also be used in place of cranial. Rostral is derived from the Latin word rostrum, meaning ‘beak’.
The dorsal surface of the embryo and the adult is the back (Figure 2.2). Dorsal also refers to the surface of the foot opposite to the plantar surface, the surface of the tongue covered with papillae, and the superior surface of the brain, so some care is needed.
The ventral surface of the embryo is the front or anterior of the embryo, opposite the dorsal surface.
As with adult anatomy, structures nearer to the midline sagittal plane are more medial, and structures further from the midline are more lateral (Figure 2.3). This also helps us describe the left–right axis of the embryo.
Proximal and distal are a little different from medial and lateral, but similarly describe structures near to the centre of the body (proximal) and further from the centre (distal) (Figure 2.1). These terms are typically used to describe limb structures. The hand is distal to the elbow, for example.
Often, to show the parts of the embryo being described, illustrations must be of a section of the embryo or a structure. These sections may be transverse, median, coronal or oblique. You can see these planes of se ations on the opposite page (Figures 2.4–2.6).