TISSUES OF THE BODY. 



269 



description was held to be correct. According to him, the first item in the develop- 

 ment is the formation of a groove in the edges of the jaws, taking place in the human 

 embryo during the sixth week. In this the twenty-teeth germs of first dentition take 

 their rise. He supposed hollows to be formed around these by the subsequent develop- 

 ment of septa between the several dental germs, and that these underwent later on a 

 closure above. This theory of Goodsir was attacked most vigorously, at a later date, 

 in the works of French histologists, Ghiillot, Magitot, and Robin. According to the 

 latter, the tooth sacs, dental germs, and remaining parts, are developed, in the first 

 instance, within the sub-mucous connective-tissue, quite independent of epithelium 

 and mucosa. 3. Huxley was the first to declare the whole enamel organ to be of 

 epithelial origin. 4. In the fifth month of intra-uterine life there may already be 

 seen new follicles, situated above the germs of the milk-teeth in an oblique position. 

 They become, however, more vertical later on, and lie behind and beneath the milk- 

 teeth. Their ossification is spread over the earlier years of infancy. Since the histo- 

 genic occurrences in both cases are similar, it will suffice if we confine ourselves in 

 the text to the consideration of the milk-teeth. 



154. 



The connective-tissue envelope of the tooth sac (fig. 258, a) consists 

 (as we have already seen in the previous section), at an early period, of 

 two layers, an external (a 1 ) and an internal (# 2 ). The first of these pre- 

 sents a great denseness in its fibrous text-are ; the latter, rich in cellular 

 elements, preserves a softer and more gelatinous character. The inner 

 surface of the dental sac assumes a more or less homogeneous aspect, and 

 to such an extent sometimes that a hyaline terminal layer has been 

 spoken of. 



The occurrence of villous projections of this inner layer, which are 

 directed towards the surface of 

 the enamel organ, is of great 

 interest. They appear to be 

 equivalent to the ordinary 

 vascular papillte of a mucous 

 membrane (1). A complex vas- 

 cular network, which receives its 

 blood from the vessels of the 

 jaws and gums, traverses the 

 whole parietal portion of the 

 dental sac, and may be seen 

 forming loops in the projections 

 just mentioned. 



The enamel organ presents for 

 our consideration, upon its con- 

 cave under surface, a coating of 

 epithelial cells already long 

 known. The latter are narrow, 

 cylindrical, and nucleated ; in 

 length 0-0226-0-0338 mm., 

 and in breadth 0'0451 mm. The 

 whole of this layer was formerly 

 called the enamel membrane. 



The epithelium, on the other 

 hand, which clothes the external 

 convex surface of the enamel organ (b), was only generally recognised at a 

 later date. It consists of low cells, measuring in man 0'0113 mm. (2). 



The last-named coating, however, does not by any means everywhere 

 possess the same thickness : it forms, rather, numerous small bud-like 



Fip. 258. Dental sac of a tolerably mature humtin 

 foetus, partly diagrammatic, a, fibrous wall of the 

 sac with its external stratum a 1 , and internal a 2 ; 

 6. enamel organ, with its papillary and parietal cells c; 

 d, enamel membrane and enamel prisms; /, dental 

 germ with its capillaries g; continuation of the con- 

 nective-tissue of the parietes into that of the dental 

 germ. 



