DEVELOPMENT OF THE HEART AND VESSELS. 103 



These hairs drop off after birth, and give place to the permanent hairs. The 

 sudoriferous and sebaceous glands are also formed from the epithelial layer 

 about the fifth and sixth month respectively. The mammary gland is also 

 formed from the deeper layer of the epithelium. Its first rudiment is seen 

 about the third month, in the form of a small projection, from which others 

 radiate, and which then give rise to the glandular follicles and ducts. The 

 development of the former, however, remains imperfect, except in the adult 

 female, and especially after pregnancy. 



The muscles become visible about the seventh or eighth week. The source 

 of their development is not completely determined, for the muscles of the 

 limbs. The vertebral muscles appear to be developed from the " muscular 

 laminae" of the primitive vertebral disks (Fig. 53, 13), and the muscles of the 

 neck and jaws, as well as those which inclose the cavities of the thorax and 

 abdomen, are also formed from the same source. They do not meet in the 

 middle line of the body till about the fourth month. The cutaneous muscles 

 are developed from the cutaneous portion of the middle blastodermic layer. 



Development of the Heart and Great Vessels. The first trace of the heart is 

 found about the tenth or twelfth day, in the form of a mass of cells proceeding 

 from the middle layer of the blastodermic vesicle, and the anterior wall of the 

 intestinal cavity. It soon forms a bent tube lying in front of the embryo, and 

 only connected to it by its vessels (Fig. 59, 14). The heart is situated at first 

 at the anterior end of the embryo, lying opposite the last two cerebral vesicles. 

 As the head is developed, the heart falls as it were backwards to the lower part 

 of the neck, and then to the thorax. It fills the whole thoracic cavity about 

 the second month. As the lungs and thoracic parietes form, the heart assumes 

 its permanent position. The tube is soon curved into the shape of the letter 

 S, the arterial part being situated above, in front, and to the right, the venous 

 below, behind, and to the left. Traces of the auricular appendages are early 

 perceptible on the venous part. Then the walls of the ventricular portion begin 

 to thicken in regard to the auricular part. The ventricle is separated by a con- 

 striction from the dilated part above, which corresponds to the aortic sinus or 

 bulb (Fig. 64, 1), and from the posterior or auricular dilatation. Then each of 



Fig. 64. 



fl 



Heart at the fifth week. A. Opened from the abdominal aspect. 1. Arterial sinus. 2. Aortic arches 

 uniting behind to form the descending aorta. 3. Auricle. 4. Auriculo-ventricular orifice. 5. Commencing 

 septum ventriculorum. 6. Ventricle. 7. Inferior vena cava. B. Posterior view of the snme. 1. Trachea. 

 2. Lungs. 3. Ventricles. 4, 5. Auricles. 6. Diaphragm. 7. Descending aorta. 8, 9, 10. Pneumogastrio 

 nerves and their branches. 



these three parts becomes subdivided by a septum. After the completion of 

 the ventricular septum the auricular is commenced. The septum ventriculorum 

 is at first almost transverse, and divides oft' a smaller portion (the right ven- 

 tricle) from the common cavity. This septum is complete about the eighth 

 week, and then the interauricular begins to grow, commencing from above and 

 behind, and coalescing with the edge of the interventricular septum, so as to 

 leave an orifice (auriculo-ventricular) on either side. The auricular septum, 



