Prenatal Development of Perineal Musculature
At 6 weeks, the flexed limbs have not yet rotated out of their primary position (see Plate 1-8). Because the upper and lower limbs later undergo opposite rotations to reach…
Cross Sections of Body at 6 to 7 Weeks
In humans, a striated cloacal sphincter muscle and levator ani muscle (pelvic diaphragm) arise from the third sacral to the first coccygeal myotomic hypomeres and are well developed by the…
Development of Skeletal Muscle Fibers
After the transverse processes appear, the ribs form in the sclerotomal tissue that extends by differentiation into the ventral portions of the original clefts between the somites. The maximum development…
Segmental Distribution of Myotomes in Fetus of 6 Weeks; Developing Skeletal Muscles at 8 Weeks
2. Portions of successive myotomes commonly fuse to form a composite single muscle (the erector spinae and rectus abdominis muscles). The latter is formed by the fusion of the ventral…
Remodeling: Maintenance of Basic Form and Proportions of Bone During Growth
The cells of the myotome, the mononucleated myoblasts, elongate in a direction parallel to the long axis of the embryo (see Plate 1-17) and undergo repeated mitotic divisions, subsequently fusing…
Development of Three Types of Synovial Joints
During vertebrate evolution, the head underwent changes related to the development of the special senses. The anterior end of the nerve cord became a brain, and the nerves passing to…
Growth in Width of a Bone and Osteon Remodeling
The dense concentration in cardiac muscle of orderly arrangements of interdigitating actin and myosin molecules, which could synchronously slide across each other throughout the atrial or ventricular muscle, resulted in…
Progressive Stages in Formation of Vertebral Column, Dermatomes, and Myotomes; Mesenchymal Precartilage Primordia of Axial and Appendicular Skeletons at 5 Weeks
The branchial arch skeleton is derived from the embryonic counterparts of the gill arches that support the mouth and pharynx of present-day adult fish and tailed amphibians. The most primitive…
Growth and Ossification of Long Bones
MUSCLES Characteristically, all living cells, including protozoa and slime molds, contain the contractile proteins actin and myosin. Thus, actin and myosin are present in all the cells of the human…
Secondary Osteon (Haversian System)
Restriction of joint motion by permanent paralysis early in development can result in the loss of the joint cavity by having a permanent fusion occur between the apposed surfaces of…
