108 GENERAL ANATOMY. 



testine. During the fourth week these depressions are found on either side, 

 opening freely into the pharynx, and from the original pouches other secondary 

 pouches are given off, so that by the eighth week the form of the lobes of the 

 lungs may be made out. The two primary pouches have a common pedicle of 

 communication with the pharynx. This is developed into the trachea (Fig. 

 64), the cartilaginous rings of which are perceptible about the seventh week. 

 The parts which afterwards form the larynx are recognized as early as in the 

 sixth week, viz : a projection on either side of the pharyngeal opening, the 

 rudiment of the arytenoid cartilages, and a transverse elevation from the third 

 pharyngeal arch, which afterwards become the epiglottis : the vocal cords and 

 ventricles of the larynx are seen about the fourth month. The traces of the 

 diaphragm appear early, in the form of a fine membrane, separating the lungs 

 from the Wolffian bodies, the stomach and liver, but the source of its formation 

 has not been ascertained. The pleural and peritoneal cavitjes are then sepa- 

 rated, having been common up to this time. The serous membrane of the 

 pleura is formed about the tenth week ; but its development is also unknown. 



Development of the Genito -urinary Organs. The allantois communicates at 

 first with the lower part of the primitive intestine by a canal the urachus. 

 After the second month the lower part of the urachus dilates, so as to form the 

 bladder, which then communicates above with the cavity of the urachus, and 

 below with the rectum, by a canal of communication which is afterwards trans- 

 formed into the urethra. The urachus is obliterated before the termination of 

 foetal life; but the cord formed by its obliteration is perceptible throughout 

 life, passing from the upper part of the bladder to the umbilicus. 



The kidneys are also formed from the lower end of the urachus. They are at 

 first hollow organs lying behind and below the Wolffian body. As their dis- 

 tance from the bladder increases, the ureters become developed, and the simple 

 cul-de-sacs in which the fcetal kidneys commence, divide and subdivide so as to 

 form lobulated organs provided with calices in their interior. This lobulation 

 is perceptible for some time after birth. 



The suprarenal bodies are formed independently both of the kidneys and 

 Wolffian bodies. 



The Wolffian body, or primordial kidney, is perceptible about the third week, 

 forming a mass of cells which soon give rise to a hollow organ, situated on 

 either side of the primitive vertebrae, and extending from the heart to the lower 

 end of the embryo, terminating above in a cul-de-sac and opening below into 

 the bladder. The structure of the Wolffian body is in many respects analogous 

 to that of the permanent kidney. It is composed partly of an excretory canal 

 into which open numerous "conduits," rectilinear at first, but afterwards tor- 

 tuous, and partly of a cellular or glandular structure, in which Malpighian tufts 

 are found. It is fixed to the diaphragm by a superior ligament, and to the 

 spinal column by an inferior or lumbar ligament. Its office is the same as that 

 of the kidneys, viz., to secrete fluid containing urea, which accumulates in the 

 bladder. When the permanent kidneys are formed, the greater part of the 

 Wolffian body disappears. The rest takes part in the formation of the genital 

 organs. 



The internal genital organs have at first no distinctive signs of sex. They are 

 developed from the Wolffian body, the genital gland, and the conduit of Mliller. 

 The genital glands are masses of cells which are formed towards the sixth week 

 of fcetal life. They are produced from the middle blastodermic layer, and lie 

 on the inside of the Wolffian body, to which they are attached by a mesenteric 

 layer of peritoneum. The conduit of Miiller, or genital duct, is formed at the 

 same time as the genital gland, and like it from the middle blastodermic layer. 

 It is at first a mere cellular cord, and then represents a canal, the upper part 

 of which is closed; the lower opens into the bladder. It lies internal and 

 anterior to the duct of the Wolffian body. 



