BLOOD-VASCULAR SYSTEM, ETC., OF AMIURUS CATUS. 433 



In a section through the spleen of a young fish, (one year old, judg- 

 ing by its size), it is seen to be surrounded by a delicate connective 

 tissue capsule (PI. VIII., Fig. 4, c). At a few points delicate pro- 

 cesses pass into the substance of the gland. In the gland substance 

 the Malpighian corpuscles (Fig. 4, m.c.') varying in size and foi-ra 

 according to the direction in which they are cut, occur evenly and 

 thickly throughout, stirrounded by a very openly reticulate pulp tis- 

 sue. The larger veins and arteries lie together, and in many in- 

 stances the artery lies wholly within the lumen of the vein, appear- 

 ing as if attached to the inner surface of its wall. A. most noticeable 

 feature is the small patch of brown pigment in the majority of the 

 Malpighian corpuscles to which they are strictly confined, never being 

 found in the pulp. 



When we examine the pulp-tissue with a power of about 600 dia., 

 it is seen to consist of large plate-like nucleated cells, (PL VIII, Fig. 

 7, a) which unite with one another by branched processes enclosing 

 large vesicular spaces (PI. VIII., Fig. 5, v. s.) To their surfaces are 

 attached a few lymphoid cells similar to those of the corpuscle, be- 

 sides adherent blood-cells. This reticulate tissue is continued through 

 the corpuscles and attached to the vessels, although this is difficult to 

 make out, because in the Malpighian corpuscles the spaces are almost 

 completely filled with lymphoid cells, except next the artery 

 '{Fig. 5, x), where there are often spaces as in higher forms. 



The lymphoid cells of the Malpighian corpuscles vary greatly in 

 size and shape (Fig. 7, d), but the bulk of the tissue is made up of 

 very small cells with a nucleus which nearly fills the interior. This 

 tissue seems to accompany and surround all the branches of the 

 artery. 



The brown pigment consists of amorphous granules which may at- 

 tain a size of 12.4 /i, but are usually smaller. These pigment granules 

 are formed in cells which when full of pigment measure about 15'5/it. 

 It is only in a few cases that the surrounding cell can be seen ; as a 

 rule it has disappeared, leaving the granules adherent in a mass, 

 (Fig. 7, b) or allowing them to be scattered in the tissue, (Fig. 5, g). 



So marked is the difference between a section of the spleen of a 

 young fish and that of an old one, that at first sight they would 

 -scarcely be recognized as from the same animal. The place occu- 

 pied by the pulp (Fig. 4), has been filled by a dense connective tis- 

 .sue stroma which divides the gland into lobules as seen in section. 



