DEVELOPMENT OF THE THYMUS. 109 



central yolk-cells, which lie free in the cavity, and are not attached to 

 its internal surface, gradually, and at length completely, disappear. They 

 are not directly converted into the cells of the mucous membrane of the 

 intestine. 



With regard to the first formation, and subsequent development of the 

 intestinal system in Mammalia, the account given by Bischoff is in close ac- 

 cordance with that furnished by Von Baer.* (See fig. 24.) The process 

 pursued is very similar to that which takes place in the development of the 

 intestine in the chick. ] 



Digestive Glands. The account given by Professor Miiller]; of the mode 

 of development of the large glands opening into the intestinal canal, as 

 the liver and pancreas in birds, has been for the most part confirmed by 

 Bischoff, in the case of Mammalia. The salivary glands also pursue a 

 similar mode of development. || 



Development of the Respiratory Apparatus.^ 



Thymus Gland. The development of the thymus gland has been 

 investigated by Mr. Simon.** The earliest form in which he has dis- 

 covered it, in the embryos of swine and oxen (on which animals his re- 

 searches were, for the most part, made), is that of a simple tube, lying 

 along the carotid vessels, and surrounded by faint indications of nascent 

 areolar tissue. The contents of the tube are granular and dotted; its 

 membrane is constituted of a fine, transparent, homogeneous tunic, pre- 

 senting, at regular intervals, slight elongated thickenings of its substance, 

 which are probably the remains of nuclei of primordial cells from the 

 coalescence of a linear series of which it is most likely the tubule is 

 originally formed. The second stage in the process of development is 

 very analogous to the mode of growth attributed to true glands : the tube 

 bulges at certain points of its length on one side or the other, and gives 

 origin to diverticula or follicles, which maintain their connection with 

 its cavity; and are filled with the same contents and bounded by the 

 same transparent membrane as the tubule itself. Slight differences are 

 observed in the mode in which these diverticula are formed, and in the 

 rapidity with which the process takes place at different parts of the gland, 

 but they always tend to assume a more or less spherical form, and to 

 retain their connexion with the main canal by means of a narrow isthmus 

 of communication. In the further growth of the gland secondary and 

 tertiary hollow projections extend from each of the primary follicles, and 

 by a continuation of the process, new groups of follicles are successively 



* Miiller's Physiology, p. 1568, and fig. 208. f Ibid. p. 1540. 



Physiology, vol. i. p. 489. Op. cit. p. 322. || Page 323. 



f Miiller's Physiology, p. 1634. 



** A Physiological Essay on the Thymus Gland. London, 1845, 4to. 



