THE FCETUS. 1049 



The enamel is derived from the deep cells of its germ, which are elongated 

 and prism-shaped, and are calcified on becoming applied to the surface of the 

 ivory. 



The cementum is developed at the expense of the walls of the follicle, accord- 

 ing to the mode of ossification of the connective tissue. 



Eruption. — As the ivory is formed, the tooth increases in length and presses 

 the enamel germ upwards ; the latter, constantly compressed, becomes atrophied, 

 and finally disappears when the tooth has reached the summit of the follicle. 

 In the same way the young organ pierces the dental follicle and gum, and makes 

 its eruption externally. 



Such is the mode of development of the deciduous teeth. The permanent 

 ones are formed in the same manner. 



The enamel organ proceeds from a point adjoining the summit of the follicle 

 of the temporary tooth, and buries itself beneath the latter, where it forms the 

 dental bulb and the walls of the new follicle. The follicles of the three last 

 molars — which are permanent — arise independently of those of the temporary 

 teeth. The bud of the first of these molars springs from the epithelial ridge on the 

 border of the jaws, the buds of the other two being derived from the first, 



3. The liver. — This gland commences to be developed very early in all the 

 species. It appears on the surface of the duodenum in the form of two or more 

 buds, according to the number of lobules in the adult liver. To these external 

 buds are corresponding internal ones, arising from the intestinal epithelium — 

 that is, the inner lamina of the blastoderm. The first are contained in a layer of 

 the mesoblast that separates the anterior 



aditus of the pericardium ; they grow and ^^^^^^^^|^^ 

 envelop the omphalo-mesenteric vein ; 

 the second ramify in their interior, and 

 form the system of biliary canals. 



The liver grows rapidly, and, towards 

 the third month, almost entirely fills the 

 abdominal cavity ; at a later period its 

 growth is less marked, although at birth 

 it is yet proportionally larger than in origin op the liver from the intestinal 

 adult life wall in the embryo of the fowl, on the 



„ ■ „i £ X fourth day of incubation. 



4. Pancreas. — The pancreas first ap- „ , . . .. 



,., ,, ,. 1 T • ii ,- a,Heart; ft, intestine; c, everten portion, giving 



pear, hke the salivary glands, m the form oHgin to liver; d, liver; e, portion of vitel- 

 of a solid cellular bud, which afterwards line vesicle. 

 is channeled into ramescent cavities. 



5. Spleen. — According to Bischoff, this body is developed — during the second 

 month — on the large curvature of the stomach. Arnold states that it is formed 

 at the same time as the liver, in a strip extending from the stomach to the 

 duodenum. It subsequently separates from the pancreas and becomes fixed to 

 the stomach, where its elements assume the character of spleen-tissue. 



Development of the Genito-Urinary Apparatus. 



The development of the genital organs is related to that of the urinary organs, 

 as the apparatus they form have some parts in common. 



Immediately after the formation of the intestines, the genito-urinary organs 

 are furnished by the Wolffian bodies. These — also named i[\Q primordial kidneys 



