350 THE CAT. [CHAP. x. 



The LARYNX first becomes visible by the rudiments of the cartilages 

 which appear about the primitive glottis, the arytenoids seeming to 

 be the first, and the epiglottis the last to appear. 



The SPLEEN is formed entirely from the mesoblast, a thickening 

 of which appears on the left side of the stomach, near the pancreas. 

 No prolongation from the alimentary canal ever enters it, or even 

 appears in connexion with it. 



The THYROID BODY arises as a small diverticulum, lined with hypo- 

 blast, from the anterior wall of the pharynx, and consists of isolated 

 vesicles, within which are rounded cells, the vesicles multiplying by 

 constriction and division or by budding out and separation of 

 such protruded parts. It is larger relatively in the foetal and young 

 condition than in adult age. 



The THYMUS GLAND is a body which makes its appearance as 

 a tube with granular contents, surrounded by tissue into which 

 small vesicles bud forth, laterally, from it which vesicles themselves 

 again branch. The thymus is relatively larger at birth than subse- 

 quently but it still remains of large size, even at maturity. 



16. The first URINARY ORGANS which make their appearance 

 in the embryo are not the kidneys, but certain temporary urinary 

 glands called the WOLFFIAN BODIES (Fig. 151), which early attain 

 a large relative size. Afterwards their place is taken by the true 

 kidneys. The Wolffian bodies entirely losing all secreting power, and 

 more or less aborting, become (in one or the other sex) either im- 

 portant or merely functionless appendages of the generative organs. 



The Wolffian bodies make their appearance in the embryo (at a 

 time when the intestinal canal still communicates widely with the 

 umbilical vesicle) as a slight ridge on each side of the primitive 

 mesentery, and each body grows to be a reddish oblong mass beside 

 the vertebral column, reaching from the heart to the hinder end of 

 the abdomen. It consists of a series of tubes all opening into a 

 duct which runs along the outer border of the Wolffian body. Each 

 tube is somewhat convoluted and dilated at its blind end as a Mal- 

 pighian body, into which a vascular glomerulus enters, just as in 

 the true kidney the Wolffian bodies being very vascular. The 

 ducts of the two Wolffian bodies open near together into the root 

 of the allantois. 



The first parts to be developed of the "Wolffian bodies are their 

 ducts, which appear to arise from a series of invaginations (from the 

 primitive pleuro-peritoneal cavity) into that part of the mesoblast 

 which lies in the angle between the diverging somatopleure and 

 splanchno-pleure. The blind ends of these invaginations grow out 

 laterally and coalesce, so as to form a rod traversing the mesoblast 

 antero-posterioiiy, while the invaginations, which originally led 

 from the peritoneal sac, close up and disappear. The central part 

 of the rod developes a cavity, and thus the tube of the Wolffian 

 ducts comes to be formed. After this, diverticula bud off laterally 

 from this duct, and so the tubes which end in the Malpighian 

 bodies come to be formed. 



