SECT. 183.] DEVELOPMENT OF THE THYMUS. 4OI 



spherical bodies resulted, and these represented the lobules. On 

 this view, the primitive tube would be converted into the central 

 cavity of the thymus, and each outgrowth of it would become 

 transformed into an entire lobule of the organ. — In the human 

 embryo of the seventh week, I have observed the thymus lobulatcd 

 at its inferior extremity, and simple at its superior. In an embryo 

 ten weeks old, the superior end was a delicate-walled tube, o - 04'" 

 to o*o6"' in diameter, filled with polygonal cells; and at the lower 

 part (o , i6"' in diameter), there were separate, roundish outgrowths, 

 of o - o2'" to 003'" in thickness, which were partly isolated, partly 

 arranged in groups of from two to five together. The thicker and 

 low est part of the gland was entirely beset with lobules in a further 

 stage of development, which measured 008'" to 01'", and on these, 

 again, were seen the simple gland-granules, or acini, each having a 

 structureless coat aud a cellular interior. In the twelfth week I 

 did not find the thymus much larger, but the whole organ had 

 become equally beset with lobules, measuring a* 12"' to cri4"'. 

 From these observations, although the first stages have not yet 

 been seen in man, there cannot be a doubt that the gland is 

 developed in him exactly as Simon has observed in mammalian 

 animals. 



The subsequent development of the thymus presents some other 

 interesting conditions. In the embryo, it continues to grow slowly 

 from the third month onwards, extends as far as the thyroid in the 

 sixth month, and, from the seventh month onwards, contains a 

 whitish juice. After birth it does not cease growing, as was for- 

 merly believed, but it increases very considerably soon after birth, 

 and goes on growing up to the second year. From this period 

 onwards it becomes no larger, but generally remains unaltered for 

 some time longer, until at last, it wastes away and finally dis- 

 appears. The period at which these final alterations take place is 

 very various. Simon says that atrophy commences between the 

 eighth and twelfth years; but my own observations, confirming 

 those of Ecker, show that this statement cannot be regarded as 

 universally true, seeing that the thymus is frequently found well 

 nourished and distended with fluid, in the twentieth year, having 

 experienced no metamorphosis, and having just the same structure 

 as in children. It is still more difficult to ascertain the period of 

 its complete disappearance, and no definite age can be assigned for 

 the occurrence of this change, although the thymus is not, as a 

 rule, found after the fortieth year. Its disappearance is effected 

 by gradual absorption of its acini, and the simultaneous develop- 



D D 



