Respiration in Invertebrate Animals. 253 



which may not incorrectly be said to represent the lobuli or 

 acini of the glands of the vertebrated animal; and, secondly, 

 the intermediate stromatous and cellular structure which fills up 

 the spaces external to and between the vesicles. In the recent 

 gland these two parts are so evidently distinct, and indeed so 

 easily distinguishable, that they cannot be confounded. The 

 cells (B, h) ivithin the vesicles are densely crowded with semi- 

 fluid albumen-looking contents, of low refractive power. The 

 substance contained in these intravesicular cells is unques- 

 tionably the secretion of this gland. There it is, directly imder 

 the eye, in its very place of production. If by micro-chemical 

 analysis its composition while thus isolated in cells could be 

 determined, the problem as to the real nature and office of this 

 gland might indeed very readily be solved ; but the minuteness 

 of the quantity thus presented to the eye renders such a deter- 

 mination impossible. 



The nucleated cells {c) which occupy the interspaces between the 

 csecal vesicles are much larger than those contained within these 

 parts; they are densely filled with oleous granules {d) of a green- 

 ish-yellow colour and of high refractive index. The nucleus in 

 these cells is filled only with an albuminous formless semifluid 

 substance. In this respect they will be found to contrast stri- 

 kingly with the similarly placed cells of the "colour-gland" 

 afterwards to be described. These extra-follicular cells, so densely 

 charged with a secreted product, perform obviously an import- 

 ant part in the office of these glands. They are separated from 

 those within the vesicles only by the walls of the follicles (B, e) 

 themselves. These walls consist literally of only a hyaline 

 membrane, structureless, answering simply and exclusively the 

 mechanical purpose of a limitary or circumscriptive sac. No 

 cell -elements are contained in its substance. It is evident there- 

 fore that the cells are the real factors in the act of secretion. 

 The large pregnant cells (B, e), which stand on the outside of 

 the csecal follicles of the gland, are soaked in the circulating 

 fluid. It surrounds them on all sides. But the blood does not 

 penetrate in mass into the interior of the vesicles. The cells 

 therefore by which these vesicles are filled cannot derive their 

 contents directly from the blood. From the relative position of 

 these parts — the blood, the extra-follicular or stromatous cells, 

 and the intra-follicular cells, — it seems highly probable that the 

 blood is first subjected to the agency of the externally situated 

 cells which cluster around the grape-shaped ends of the glands, 

 and that the prepared contents of these cells pass thence by 

 endosmose into the interior of the follicles, where they for the 

 second time conduce to form, and where they undergo the 



