156 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



THE DEVELOPMENT OF THE REPRODUCTIVE ORGANS. 

 GENERAL DEVELOPMENT. 



In well-advanced embryos taken in January (for stage of development, compare 

 cut 38) a very minute cluster of cells can be detected on either side of the middle line 

 close upon the mesodermic partition which screens the heart from the intestine. 

 These cells are mesoblastic in origin; they possess oval or spherical nuclei which, 

 however, are not conspicuous for their size. At the time the embryo is about to hatch 

 there is less doubt in the identification of the reproductive organ (fig. 116, plate 36). 

 It now consists of a small oval, somewhat flattened mass of cells, lying close upon 

 the mesentery, next to the intestine. It appears to arise as a proliferation of the 

 mesoblast of the mesentery, but at this time is very distinct from it. 



Later, in the first and second larval stages, the reproductive organ is a more com- 

 pact, almost spherical, cell mass (about 4 \- mm. in diameter). Its position, close to the 

 anterior end of the heart, but in contact with the mesentery, is well shown in fig. 174, ov, 

 plate 43. It is now differentiated into two kinds of cells: (1) Central cells with large 

 nuclei; (2) peripheral cells with much smaller nuclei (fig. 117, plate 36). The latter 

 probably give rise to the ovarian wall, the former to the ova and follicular epithelium. 

 The clearer central cells contain a distinct reticulum in which masses of chromatin 

 are held. The organ is delicately suspended to the side of the mesentery by connective 

 tissue. I did not distinguish the outlines of cells in any part of it. 



In as late as the sixth or seventh stages the reproductive organ is still of very 

 small size and not readily seen. 



THE OVARY. 



In a female 44 mm. long (No. 2, table 32) the ovary was of the size shown in fig. 

 131. I did not observe the ducts, probably because of the poor condition of the 

 specimen when dissected. These were undoubtedly present, since their openings are 

 visible in the eighth stage (fig. 89, plate 32 — No. 3, table 34), when the animal is less 

 than an inch long. This ovary was 15 mm. long, and each lobe was about one-fourth 

 mm. in diameter. The anterior lobes embrace the masticatory stomach, and one of 

 the posterior lobes was branched. 



If the condition of the tissue could be trusted — it was preserved in alcohol, 

 considerably diluted — the organ now consisted of a distinct connective tissue wall and 

 an inclosed mass of large cells, which are the ova (fig. 146, plate 40). There was no 

 plaited or folded ovarian epithelium such as we see at a later stage. 



In a young female 2|f inches long the ovary had the size and appearance shown in 

 fig. 132. It is about 40 mm. long and has a diameter of 0.5 mm. It is opaque white. 



In a lobster 4f\ inches in length (No. 42, table 20) the ovary has the same appear- 

 ance but is somewhat larger. Its structure is now much more complex than at any of 

 the stages described. It consists of a thin connective tissue envelope and a compact 

 stroma. Folds of epithelium dip down from the surface and penetrate the interior of 

 the organ, thus dividing up the outer portions into radial compartments, in which the 

 larger eggs are seen. These contain large nuclei, with one, two, or more nucleoli. 

 The axis of the ovary lies in a stroma in which all stages in the development of 

 ova can be traced. Karyokinetic figures of dividing cells are not infrequently seen. 

 Blood now penetrates to the ovary by sinuses which come in from the wall along 

 reentrant folds of epithelium. 



