380 Royal Society. 



early period this closely corresponds with the structure of the supra- 

 renal and thyroid glands at the earliest stages of their evolution, 

 consisting of nuclei, nucleated vesicles, and a fine granular plasma, 

 the former forming a very considerable portion of its structure. 

 When the splenic vessels are formed, many of these nuclei are sur- 

 rounded bv a quantity of fine dark granules arranged in a circular 

 form, and these increase up to the time when the splenic vein is 

 formed, when nearly the whole mass is composed of nucleated vesi- 

 cles, the nuclei of which gradually break up into a mass of granules 

 which fill the cavities of the vesicles. The Malpighian vesicles are 

 developed in the pulp by the aggregation of nuclei into circular 

 masses, around which a line membrane soon appears, in a manner 

 precisely similar to those of the suprarenal and thyroid glands, with 

 which they bear the closest analogy. 



The author then traces out the development of the supra-renal 

 glands, and shows the close analogy that exists between them, the 

 spleen, and thyroid, from the similarity which their structure pre- 

 sents at the earliest period of their evolution with those glands, and 

 from the development of the several tissues following the same 

 stages in all. 



They are shown to arise on the 7th day as two separate masses 

 of blastema, situated between the upper end of the Woolfian bodies 

 and the sides of the aorta, being totally independent (as concerns 

 their development) of those bodies, or of each other. At this period 

 their minute structure bears a close resemblance to that of the 

 spleen, consisting of the same elements as that gland, excepting in 

 the existence of more numerous dark granules, which give to the 

 organ at a later period an opake and darkly granular texture. The 

 gland tissue of the organ, in the form of large vesicles, makes its 

 appearance on the 8th day, whereas in the spleen it did not exist 

 until near to the close of incubation, an interesting fact in con- 

 nection with the function of the former gland, which is mainly 

 exercised during foetal life, whilst the spleen exerts its function 

 mainly in adult life ; hence the difference in the development of the 

 tissues at different periods. The manner in which this tissue is 

 developed is similar to that by which the gland tissue of the 

 spleen was formed, viz. by an aggregation of nuclei into circular 

 masses, around which a limitary membrane ultimately forms : these 

 are first grouped together in a mass, without any subdivision into 

 cortical and medullary portions. On the 14th day the first trace of 

 this subdivision becomes manifest, by the vesicles being aggregated 

 into masses which radiate from the circumference towards the centre 

 of the gland, in some cases complete tubes being formed by the 

 junction of the vesicles, as indicated by hemispherical bulgings along 

 their walls. At a later period the organs increase in size, they 

 attain their usual position, and a more complete subdivision into 

 cortical and medullary portions is new observed. 



The author lastly traces out the development of the thyroid 

 glands, and shows the great similarity that exists between them, the 

 spleen and supra-renal glands, from the similar structure they pre- 



