THE CORPUS LUTEUM OF PREGNANCY IX SWINE. 83 



numerous that there is practically no endoplasm. Fat is present in greal quantity in the 

 lutein cells, as in the next stage, under which it will be described. In the interstitial con- 

 nective-tissue cells of the corpus luteum there is a moderate quantity of fat in fine granules. 



Second period: height of exoplasmic development. Pigs 20 to 30 nun. long, approximate 

 duration of pregnancy 25 to 30 days (fig. 5). The striking feature of this stage is that 

 all the lutein cells are in the same state. All tend to be rounded in outline, all are of the 

 same size, and show a uniform development of the exoplasmic apparatus, with ring-forms 

 in every cell. The endoplasm is still very limited in amount. The nucleus is not so chro- 

 matic as in the previous period. Osmic-blackening substance is present in considerable 

 amount in the lutein cells, in some cases almost filling the cells with globules of diameters 

 varying from 2 to 5 microns. In the most typical cases the globules are thickly clumped 

 at one end of the cell, or at both, but there is often a cluster of fat globules about the nucleus; 

 indeed, at this stage the distribution of fat is almost general, at least in the lutein cells. 

 However, the connective-tissue cells show little fat; in osmic preparations they show at 

 most a few small black granules (fig. 6). 



Third period: height of exoplasmic development, second part. — Pigs 30 to 55 mm. long, 

 approximate duration of pregnancy 30 to 40 days (fig. 7). The previous stage passes 

 insensibly into this, in which in many cells the exoplasmic structure is more varied in form; 

 some cells contain the ring-forms (fig. 7, a), others present irregular channels and clefts 

 (fig. 7, b). The endoplasmic zone about the nucleus can now be seen, and in a very few 

 cells reaches to the periphery, in which case there are no peripheral spaces in the cytoplasm. 

 In the endoplasm are seen the chromatic granules described by Cesa-Bianchi (fig. 7, c). 



Fourth period: transition. — Pigs 55 to 140 mm. long, approximate duration of preg- 

 nancy 40 to 75 days (fig. 8). The ring-structures are disappearing, and become rare in 

 the latter half of this stage. I have seen them but once or twice in ovaries from pregnancies 

 above 140 mm., and therefore set that as the upper limit of the period. It is noted that 

 the progressive changes of the cells are a little earlier at the periphery of the corpus luteum 

 than at the center, and hence estimates of the age of the corpus luteum must be based upon 

 a study of the whole area, giving more weight to the state of the peripheral tissue. In the 

 earlier half of this period one sees an occasional cell without any exoplasmic clear areas or 

 channels; in other words the entire cell, from nucleus to border, is occupied by the homo- 

 geneous endoplasm. Such cells become fairly common toward the latter half of the period, 

 and in some fields of the lens may form half of the lutein cells. Fat has decreased in quan- 

 tity in the lutein cells, where it is found chiefly at the border between endoplasm and exo- 

 plasm. In the lutein cells it occurs in globules of varying size (fig. 9, a), but in the con- 

 nective-tissue cells in smaller granules only (fig. 9, b). 



Fifth period: endoplasmic development. — Pigs 140 to 170 mm. long, approximate dura- 

 tion of pregnancy 75 to 105 days (fig. 10). A marked change has taken place in the 

 corpus luteum, although it is not strikingly shown in the figure — a change which has for 

 its most obvious feature a great increase in the diversity of the cells. Although the ring- 

 forms have disappeared from all but a rare cell or two, yet many cells show considerable 

 peripheral canalization. Next to such a cell may be a lutein cell which shows only endo- 

 plasm (fig. 10, a). Moreover, there has been an increase in the amount of connective 

 tissue, so that in many sections the lutein cells are somewhat spread apart. Between them, 

 adding to the diversity of appearance, lie the several kinds of cells which we have mentioned 

 as members of this complicated structure, namely, the branching or spindle cells of the 

 connective tissue, with their product, the reticular fibrils; the darkly staining cells which 



