DUCTLESS GLANDS IN THE CHICK. 
301 
having a quantity of fine dark granules surrounding them in a circular form. There 
are also observed many nucleated vesicles, rather larger than the blood-corpuscles, 
the nuclei in which are circular, whilst the cavity of the vesicle contains also a few 
small pale granules. Some small masses of reddish brown granules may also be ob- 
served ; they exist, however, very sparingly. From the time when the formation of 
the arteries supplying the organ is completed, up to that when the splenic vein is 
observed to be also constituted, these latter elements not only form a portion of the 
pulp, but are in fact its chief components. When, however, the splenic vein is nearly 
completed, a considerable change is observed to have occurred in the nucleated 
vesicles ; those which had previously formed only a small portion of its substance, 
now exist as the chief element, and the majority contain a nucleus with irregular 
margins. Their form is chiefly circular, their outer wall in some cases very distinct, 
in others less so, from the cell being distended with dark granules. There is gene- 
rally only a single nucleus, which has a dark outer margin, and contains either a 
nucleolus or two or three granules. In some the nucleus is of an irregular form, and 
more indistinct ; the cavity of the cell in these cases containing a few granules, as 
the nuclei become more irregular and granular; these granules increase until at last 
the nucleus appears to be entirely broken up, when they all become crowded with 
small granules. Such is the structure the pulp tissue of the spleen presents, from 
the period when the splenic vein is formed, up to the time when incubation is com- 
pleted. 
Development of the Malpighian Vesicles. 
The vesicles of the spleen are developed in a manner perfectly similar to those of 
the supra-renal and thyroid glands, with which they appear to bear a very close 
analogy ; they are not developed, however, in the chick until the period of incuba- 
tion is near to its completion. Between the twentieth and the twenty-first days there 
may be observed at the angles of division of the smaller blood-vessels, as well as 
upon the walls of the vessels themselves, rather large masses of nuclei and granules, 
arranged together in a circular form ; these masses are not, however, at this period 
enclosed by any investing membrane, but are rather intimately connected with the 
walls of the vessels, as they are not removed by delicate manipulation, and only 
when a greater amount of force is used. A few days after incubation is completed 
these vesicles are observed to be partly surrounded by a faintly delicate homogeneous 
membrane, and in about a week the vesicles are distinctly formed, and present the 
same structure as in the adult bird; they are circular or oval, varying considerably 
in size, and consisting of an outer investing membrane, pale, homogeneous, or faintly 
granular in texture, and containing in their interior a mass of nuclei and numerous 
small dark granules. 
